Phospholipase C
Encyclopedia
Phosphoinositide phospholipase C (PLC) is a family of eukaryotic intracellular enzymes that play an important role in signal transduction processes. In general, this enzyme is denoted as Phospholipase C, although three other families of phospholipase C enzymes
have been identified in bacteria
and in eukaryotic trypanosomes. Phospholipases C enzymes belong to the family of hydrolase
s and phosphodiesterases.
Phospholipase Cs participate in phosphatidylinositol 4,5-bisphosphate
(PIP2) metabolism and lipid signaling
pathways in a calcium-dependent manner. At present, the superfamily consists of six sub-families comprising a total of 13 separate isoforms that differ in their mode of activation, expression levels, catalytic regulation, cellular localization, membrane binding avidity and tissue distribution. All are capable of catalyzing the hydrolysis of PIP2 into two important second messenger molecules, which go on to alter cell responses such as proliferation
, differentiation, apoptosis
, cytoskeleton
remodeling, vesicular trafficking, ion channel
conductance, endocrine function and neurotransmission
.
at the inner leaflet of the plasma membrane into the two second messengers, inositol triphosphate
(IP3) and diacylglycerol
(DAG).
The chemical reaction
may be expressed as:
Thus, the two substrates
of this enzyme are H2O
and 1-phosphatidyl-1D-myo-inositol 4,5-bisphosphate
(PIP2, phosphatidylinositol bisphosphate), whereas its two products
are diacylglycerol and 1D-myo-inositol 1,4,5-trisphosphate
(IP3, inositol triphosphate).
PLCs catalyze the reaction in two sequential steps. The first reaction is a phosphotransferase
step that involves an intramolecular attack between the hydroxyl
group at the 2' position on the inositol
ring and the adjacent phosphate
group resulting in a cyclic IP3 intermediate. At this point, DAG is generated. However, in the second phosphodiesterase
step, the cyclic intermediate is held within the active site
long enough to be attacked by a molecule of water, resulting in a final acyclic IP3 product. It should be mentioned that bacterial forms of the enzyme, which contain only the catalytic lipase
domain, produce cyclic intermediates exclusively, whereas the mammalian isoforms generate predominantly the acyclic product. However, it is possible to alter experimental conditions (e.g., temperature, pH) in vitro such that some mammalian isoforms will alter the degree to which they produce mixtures of cyclic/acyclic products along with DAG. This catalytic process is tightly regulated by reversible phosphorylation
of different phosphoinositides and their affinity for different regulatory proteins.
) that display affinity for different phospholipid
components of the plasma membrane. It is important to note that research has also discovered that, in addition to the plasma membrane, PLCs also exist within other sub-cellular regions such as the cytoplasm
and nucleus
of the cell. At present, it is unclear exactly what the definitive roles for these enzymes in these cellular compartments are, particularly the nucleus.
Phospolipase C performs a catalytic mechanism, generating inositol triphosphate
(IP3) and diacylglycerol
(DAG).
These molecules then go on to modulate the activity of downstream proteins important for cellular signaling. IP3 is soluble, and diffuses through the cytoplasm and interacts with IP3 receptors on the endoplasmic reticulum
, causing the release of calcium and raising the level of intracellular calcium
.
Further reading: Function of calcium in humans
DAG remains tethered to the inner leaflet of the plasma membrane due to its hydrophobic character, where it recruits protein kinase C
(PKC), which becomes activated in conjunction with binding calcium ions. This results in a host of cellular responses through stimulation of calcium-sensitive proteins such as Calmodulin
.
Further reading: Function of protein kinase C
domains, and a single C2 domain that flank the X and Y catalytic core. An N-terminal PH domain is present in every family except for the sperm-specific ζ isoform.
SH2
(phosphotyrosine binding) and SH3
(proline-rich-binding) domains are found only in the γ form (specifically within the linker region), and only the ε form contains both guanine nucleotide exchange factor
(GEF) and RA (Ras Associating) domains. The β subfamily is distinguished from the others by the presence of a long C-terminal extension immediately downstream of the C2 domain, which is required for activation by Gαq subunits, and which plays a role in plasma membrane binding and nuclear localization.
,2), -ε, -ζ, and the recently-discovered -η(1,2) isoform. Depending on the specific subfamily in question, activation can be highly variable. Activation by either Gαq
or Gβγ G-protein subunits (making it part of a G protein-coupled receptor
signal transduction
pathway) or by transmembrane receptors with intrinsic or associated tyrosine kinase
activity has been reported. In addition, members of the Ras superfamily of small GTPases (namely the Ras and Rho subfamilies) have also been implicated. It should also be mentioned that all forms of Phospholipase C require calcium for activation, many of them possessing multiple calcium contact sites in the catalytic region. The only isoform that is known to be inactive at basal intracellular calcium levels is the δ subfamily of enzymes suggesting that they function as calcium amplifiers that become activated downstream of other PLC family members.
and long C-terminal extension. Gβγ subunits are known to activate the β2 and β3 isozymes only; however, this occurs through the PH domain. The exact mechanism still requires further investigation. The PH domain of β2 and β3 plays a dual role, much like PLC-δ1, by binding to the plasma membrane, as well as being a site of interaction for the catalytic activator. However, PLC-β binds to the lipid surface independent of PIP2 with all isozymes preferring phosphoinositol-3-phosphate or neutral membranes.
Members of the Rho GTPase family (e.g., Rac1, Rac2, Rac3, and cdc42
) have been implicated in their activation by binding to an alternate site on the N-terminal PH domain followed by subsequent recruitment to the plasma membrane. A crystal structure of Rac1 bound to the PH domain of PLCβ2 has been solved. The crystal structure of Like PLC-δ1, many PLC-β isoforms (in particular, PLC-β1) have been found to take up residence in the nuclear compartment. A basic amino acid region within the enzyme's long C-terminal tail appears to function as a Nuclear Localization Signal for import into the nucleus. PLC-β1 seems to play unspecified roles in cellular proliferation and differentiation.
. Absence of this enzyme in knockout specimens severely inhibits the development of B cells because the same signaling pathways necessary for antigen mediated B cell activation are necessary for B cell development from CLPs.
In B cell signaling, PI 3-kinase is recruited to the BCR early in the signal transduction pathway. PI-3K phosphorylates PIP2 (Phosphatidylinositol 4,5-bisphosphate
) into PIP3 (Phosphatidylinositol 3,4,5-trisphosphate
). The increase in concentration of PIP3 recruits PLC-γ2 to the BCR complex which binds to BLNK on the BCR scaffold and membrane PIP3. PLC-γ2 is then phosphorylated by Syk
on one site and Btk
on two sites. PLC-γ2 then competes with PI-3K for PIP2 which it hydrolyzes into IP3 (inositol 1,4,5-triphosphate), which ultimately raises intercellular calcium, and diacylglycerol (DAG), which activates portions of the PKC
family. Because PLC-γ2 competes for PIP2 with the original signaling molecule PI3K, it serves as a negative feedback
mechanism.
PLC-δ1 also possesses a classical leucine
-rich nuclear export signal
(NES) in its EF hand
motif, as well as a Nuclear localization signal
within its linker region. These two elements combined allow PLC-δ1 to actively translocate into and out of the nucleus. However, its function in the nucleus remains unclear.
The widely-expressed PLC-δ1 isoform is the best-characterized phospholipase family member, as it was the first to have high-resolution X-ray
crystal structures available for analysis. In terms of domain architecture, all of the enzymes are built upon a common PLC-δ backbone, wherein each family displays similarities, as well as obvious distinctions, that contribute to unique regulatory properties within the cell.Because it is the only family found expressed in lower eukaryotic organisms such as yeast
and slime molds, it is considered the prototypical PLC isoform. The other family members more than likely evolved from PLC-δ as their domain architecture and mechanism of activation were expanded. Although a full crystal structure has not been obtained, high-resolution X-ray crystallography
has yielded the molecular structure of the N-terminal PH domain complexed with its product IP3, as well as the remainder of the enzyme with the PH domain ablated. These structures have provided researchers with the necessary information to begin speculating about other family members such as PLCβ2.
; PLCB2
; PLCB3
; PLCB4
; PLCD1
; PLCD3
; PLCD4
; PLCE1
;
PLCG1
; PLCG2
; PLCH1; PLCH2; PLCL1; PLCL2; PLCZ1
- Mammalian and bacterial phospholipases - PH domain of phospholipase C delta 1 - α-toxin (a bacterial phospholipase C)
Phospholipase C
Phosphoinositide phospholipase C is a family of eukaryotic intracellular enzymes that play an important role in signal transduction processes. In general, this enzyme is denoted as Phospholipase C, although three other families of phospholipase C enzymes have been identified in bacteria and in...
have been identified in bacteria
Zinc dependent phospholipase C
Zinc-dependent prokaryotic phospholipases C is a family of bacterial phospholipases C, some of which are also known as alpha toxins.Bacillus cereus contains a monomeric phospholipase C of 245 amino-acid residues. Although PLC prefers to acton phosphatidylcholine, it also shows weak catalytic...
and in eukaryotic trypanosomes. Phospholipases C enzymes belong to the family of hydrolase
Hydrolase
In biochemistry, a hydrolase is an enzyme that catalyzes the hydrolysis of a chemical bond. For example, an enzyme that catalyzed the following reaction is a hydrolase:-Nomenclature:...
s and phosphodiesterases.
Phospholipase Cs participate in phosphatidylinositol 4,5-bisphosphate
Phosphatidylinositol 4,5-bisphosphate
Phosphatidylinositol 4,5-bisphosphate or PtdInsP2, also known simply as PIP2, is a minor phospholipid component of cell membranes...
(PIP2) metabolism and lipid signaling
Lipid signaling
Lipid signaling, broadly defined, refers to any biological signaling event involving a lipid messenger that binds a protein target, such as a receptor, kinase or phosphatase, which in turn mediate the effects of these lipids on specific cellular responses...
pathways in a calcium-dependent manner. At present, the superfamily consists of six sub-families comprising a total of 13 separate isoforms that differ in their mode of activation, expression levels, catalytic regulation, cellular localization, membrane binding avidity and tissue distribution. All are capable of catalyzing the hydrolysis of PIP2 into two important second messenger molecules, which go on to alter cell responses such as proliferation
Cell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...
, differentiation, apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
, cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
remodeling, vesicular trafficking, ion channel
Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...
conductance, endocrine function and neurotransmission
Neurotransmission
Neurotransmission , also called synaptic transmission, is the process by which signaling molecules called neurotransmitters are released by a neuron , and bind to and activate the receptors of another neuron...
.
Reaction and catalytic mechanism
All family members are capable of catalyzing the hydrolysis of PIP2, a phosphatidylinositolPhosphatidylinositol
Phosphatidylinositol is a negatively charged phospholipid and a minor component in the cytosolic side of eukaryotic cell membranes....
at the inner leaflet of the plasma membrane into the two second messengers, inositol triphosphate
Inositol triphosphate
Inositol trisphosphate or inositol 1,4,5-trisphosphate , together with diacylglycerol , is a secondary messenger molecule used in signal transduction and lipid signaling in biological cells. While DAG stays inside the membrane, IP3 is soluble and diffuses through the cell...
(IP3) and diacylglycerol
Diglyceride
A diglyceride, or a diacylglycerol , is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages....
(DAG).
The chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
may be expressed as:
- 1-phosphatidyl-1D-myo-inositol 4,5-bisphosphate + H2O
1D-myo-inositol 1,4,5-trisphosphate + diacylglycerol
Thus, the two substrates
Substrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
of this enzyme are H2O
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and 1-phosphatidyl-1D-myo-inositol 4,5-bisphosphate
Phosphatidylinositol (4,5)-bisphosphate
Phosphatidylinositol 4,5-bisphosphate or PtdInsP2, also known simply as PIP2, is a minor phospholipid component of cell membranes...
(PIP2, phosphatidylinositol bisphosphate), whereas its two products
Product (chemistry)
Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...
are diacylglycerol and 1D-myo-inositol 1,4,5-trisphosphate
Inositol triphosphate
Inositol trisphosphate or inositol 1,4,5-trisphosphate , together with diacylglycerol , is a secondary messenger molecule used in signal transduction and lipid signaling in biological cells. While DAG stays inside the membrane, IP3 is soluble and diffuses through the cell...
(IP3, inositol triphosphate).
PLCs catalyze the reaction in two sequential steps. The first reaction is a phosphotransferase
Phosphotransferase
Phosphotransferases are a category of enzymes that catalyze phosphorylation reactions. The general form of the reactions they catalyze is: A—P + B ⇔ B—P + A...
step that involves an intramolecular attack between the hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
group at the 2' position on the inositol
Inositol
Inositol or cyclohexane-1,2,3,4,5,6-hexol is a chemical compound with formula 6126 or 6, a sixfold alcohol of cyclohexane. It exists in nine possible stereoisomers, of which the most prominent form, widely occurring in nature, is cis-1,2,3,5-trans-4,6-cyclohexanehexol, or myo-inositol...
ring and the adjacent phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
group resulting in a cyclic IP3 intermediate. At this point, DAG is generated. However, in the second phosphodiesterase
Phosphodiesterase
A phosphodiesterase is any enzyme that breaks a phosphodiester bond. Usually, people speaking of phosphodiesterase are referring to cyclic nucleotide phosphodiesterases, which have great clinical significance and are described below...
step, the cyclic intermediate is held within the active site
Active site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
long enough to be attacked by a molecule of water, resulting in a final acyclic IP3 product. It should be mentioned that bacterial forms of the enzyme, which contain only the catalytic lipase
Lipase
A lipase is an enzyme that catalyzes the formation or cleavage of fats . Lipases are a subclass of the esterases.Lipases perform essential roles in the digestion, transport and processing of dietary lipids in most, if not all, living organisms...
domain, produce cyclic intermediates exclusively, whereas the mammalian isoforms generate predominantly the acyclic product. However, it is possible to alter experimental conditions (e.g., temperature, pH) in vitro such that some mammalian isoforms will alter the degree to which they produce mixtures of cyclic/acyclic products along with DAG. This catalytic process is tightly regulated by reversible phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
of different phosphoinositides and their affinity for different regulatory proteins.
Cell location
PLCs perform their catalytic function at the plasma membrane where their substrate PIP2 is present. This membrane docking is mediated mostly by lipid-binding domains (e.g. PH domain and C2 domainC2 domain
A C2 domain is a protein structural domain involved in targeting proteins to cell membranes. It is a beta-sandwich composed of 8 β-strands that co-ordinates two or three calcium ions, which bind in a cavity formed by the first and final loops of the domain, on the membrane binding face.-Coupling...
) that display affinity for different phospholipid
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
components of the plasma membrane. It is important to note that research has also discovered that, in addition to the plasma membrane, PLCs also exist within other sub-cellular regions such as the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
and nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
of the cell. At present, it is unclear exactly what the definitive roles for these enzymes in these cellular compartments are, particularly the nucleus.
Function
Inositol triphosphate
Inositol trisphosphate or inositol 1,4,5-trisphosphate , together with diacylglycerol , is a secondary messenger molecule used in signal transduction and lipid signaling in biological cells. While DAG stays inside the membrane, IP3 is soluble and diffuses through the cell...
(IP3) and diacylglycerol
Diglyceride
A diglyceride, or a diacylglycerol , is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages....
(DAG).
These molecules then go on to modulate the activity of downstream proteins important for cellular signaling. IP3 is soluble, and diffuses through the cytoplasm and interacts with IP3 receptors on the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
, causing the release of calcium and raising the level of intracellular calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
.
Further reading: Function of calcium in humans
DAG remains tethered to the inner leaflet of the plasma membrane due to its hydrophobic character, where it recruits protein kinase C
Protein kinase C
Protein kinase C also known as PKC is a family of enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins. PKC enzymes in turn are activated by signals such as increases in...
(PKC), which becomes activated in conjunction with binding calcium ions. This results in a host of cellular responses through stimulation of calcium-sensitive proteins such as Calmodulin
Calmodulin
Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...
.
Further reading: Function of protein kinase C
Domain structure
In terms of domain organization, all family members possess homologous X and Y catalytic domains in the form of a distorted Triose Phosphate Isomerase (TIM) barrel with a highly-disordered, charged, and flexible intervening linker region. Likewise, all isoforms possess four EF handEF hand
The EF hand is a helix-loop-helix structural domain found in a large family of calcium-binding proteins. The EF-hand motif contains a helix-loop-helix topology, much like the spread thumb and forefinger of the human hand, in which the Ca2+ ions are coordinated by ligands within the loop...
domains, and a single C2 domain that flank the X and Y catalytic core. An N-terminal PH domain is present in every family except for the sperm-specific ζ isoform.
SH2
SH2 domain
The SH2 domain is a structurally conserved protein domain contained within the Src oncoprotein and in many other intracellular signal-transducing proteins...
(phosphotyrosine binding) and SH3
SH3 domain
The SRC Homology 3 Domain is a small protein domain of about 60 amino acids residues first identified as a conserved sequence in the viral adaptor protein v-Crk and the non-catalytic parts of enzymes such as phospholipase and several cytoplasmic tyrosine kinases such as Abl and Src...
(proline-rich-binding) domains are found only in the γ form (specifically within the linker region), and only the ε form contains both guanine nucleotide exchange factor
Guanine nucleotide exchange factor
Guanine nucleotide exchange factors activate monomeric GTPases by stimulating the release of guanosine diphosphate to allow binding of guanosine triphosphate . A variety of unrelated structural domains have been shown to exhibit guanine nucleotide exchange activity...
(GEF) and RA (Ras Associating) domains. The β subfamily is distinguished from the others by the presence of a long C-terminal extension immediately downstream of the C2 domain, which is required for activation by Gαq subunits, and which plays a role in plasma membrane binding and nuclear localization.
Isozymes and activation
The Phospholipase C family consists of 13 isozymes split between six subfamilies, PLC-δ (1,3 & 4), -β(1-4), -γ(1PLCG1
Phospholipase C, gamma 1, also known as PLCG1, is a protein that in humans is encoded by the PLCG1 gene.-Function:The protein encoded by this gene catalyzes the formation of inositol 1,4,5-trisphosphate and diacylglycerol from phosphatidylinositol 4,5-bisphosphate...
,2), -ε, -ζ, and the recently-discovered -η(1,2) isoform. Depending on the specific subfamily in question, activation can be highly variable. Activation by either Gαq
Gq alpha subunit
Gq protein or Gq/11 is a heterotrimeric G protein subunit that activates phospholipase C . PLC in turn hydrolyzes Phosphatidylinositol 4,5-bisphosphate to diacyl glycerol and inositol triphosphate signal transduction pathway...
or Gβγ G-protein subunits (making it part of a G protein-coupled receptor
G protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
pathway) or by transmembrane receptors with intrinsic or associated tyrosine kinase
Tyrosine kinase
A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to a protein in a cell. It functions as an "on" or "off" switch in many cellular functions....
activity has been reported. In addition, members of the Ras superfamily of small GTPases (namely the Ras and Rho subfamilies) have also been implicated. It should also be mentioned that all forms of Phospholipase C require calcium for activation, many of them possessing multiple calcium contact sites in the catalytic region. The only isoform that is known to be inactive at basal intracellular calcium levels is the δ subfamily of enzymes suggesting that they function as calcium amplifiers that become activated downstream of other PLC family members.
PLC-β
PLC-β(1-4) (120-155kDa) are activated by Gαq subunits through their C2 domainC2 domain
A C2 domain is a protein structural domain involved in targeting proteins to cell membranes. It is a beta-sandwich composed of 8 β-strands that co-ordinates two or three calcium ions, which bind in a cavity formed by the first and final loops of the domain, on the membrane binding face.-Coupling...
and long C-terminal extension. Gβγ subunits are known to activate the β2 and β3 isozymes only; however, this occurs through the PH domain. The exact mechanism still requires further investigation. The PH domain of β2 and β3 plays a dual role, much like PLC-δ1, by binding to the plasma membrane, as well as being a site of interaction for the catalytic activator. However, PLC-β binds to the lipid surface independent of PIP2 with all isozymes preferring phosphoinositol-3-phosphate or neutral membranes.
Members of the Rho GTPase family (e.g., Rac1, Rac2, Rac3, and cdc42
CDC42
Cell division control protein 42 homolog also known as CDC42 is a protein involved in regulation of the cell cycle. In humans, CDC42 is encoded by the CDC42 gene.- Function :...
) have been implicated in their activation by binding to an alternate site on the N-terminal PH domain followed by subsequent recruitment to the plasma membrane. A crystal structure of Rac1 bound to the PH domain of PLCβ2 has been solved. The crystal structure of Like PLC-δ1, many PLC-β isoforms (in particular, PLC-β1) have been found to take up residence in the nuclear compartment. A basic amino acid region within the enzyme's long C-terminal tail appears to function as a Nuclear Localization Signal for import into the nucleus. PLC-β1 seems to play unspecified roles in cellular proliferation and differentiation.
PLC-γ
PLC-γ (120-155kDa) is activated by receptor and non-receptor tyrosine kinases due to the presence of two SH2 and a single SH3 domain situated betwixt a split PH domain within the linker region. Although this particular isoform does not contain classic nuclear export or localization sequences, it has been found within the nucleus of certain cell lines. There are two main isoforms of PLCγ expressed in human specimens, PLC-γ1 and PLC-γ2.PLC-γ2
PLC-γ2 plays a major role in BCR signal transductionSignal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
. Absence of this enzyme in knockout specimens severely inhibits the development of B cells because the same signaling pathways necessary for antigen mediated B cell activation are necessary for B cell development from CLPs.
In B cell signaling, PI 3-kinase is recruited to the BCR early in the signal transduction pathway. PI-3K phosphorylates PIP2 (Phosphatidylinositol 4,5-bisphosphate
Phosphatidylinositol 4,5-bisphosphate
Phosphatidylinositol 4,5-bisphosphate or PtdInsP2, also known simply as PIP2, is a minor phospholipid component of cell membranes...
) into PIP3 (Phosphatidylinositol 3,4,5-trisphosphate
Phosphatidylinositol (3,4,5)-trisphosphate
Phosphatidylinositol -triphosphate , abbreviated PIP3, is the product of the class I phosphoinositide 3-kinases phosphorylation on phosphatidylinositol -bisphosphate .-Discovery:...
). The increase in concentration of PIP3 recruits PLC-γ2 to the BCR complex which binds to BLNK on the BCR scaffold and membrane PIP3. PLC-γ2 is then phosphorylated by Syk
Syk
Spleen tyrosine kinase, also known as Syk, is an enyzme which in humans is encoded by the SYK gene.- Function :SYK, along with Zap-70, is a member of the Syk family of tyrosine kinases...
on one site and Btk
Bruton's tyrosine kinase
Bruton's tyrosine kinase is a type of kinase enzyme implicated in the primary immunodeficiency disease X-linked agammaglobulinemia . Its exact mechanism of action remains unknown, but it plays a crucial role in B cell maturation as well as mast cell activation through the high-affinity IgE receptor...
on two sites. PLC-γ2 then competes with PI-3K for PIP2 which it hydrolyzes into IP3 (inositol 1,4,5-triphosphate), which ultimately raises intercellular calcium, and diacylglycerol (DAG), which activates portions of the PKC
Protein kinase C
Protein kinase C also known as PKC is a family of enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins. PKC enzymes in turn are activated by signals such as increases in...
family. Because PLC-γ2 competes for PIP2 with the original signaling molecule PI3K, it serves as a negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
mechanism.
PLC-δ
The PLC-δ subfamily consists of three family members, δ1, 2, and 3. PLC-δ1 (85kDa) is the most well understood of the three. The enzyme is activated by high calcium levels generated by other PLC family members, and therefore functions as a calcium amplifier within the cell. Binding of its substrate PIP2 to the N-terminal PH domain is highly specific and functions to promote activation of the catalytic core. In addition, this specificity helps tether the enzyme tightly to the plasma membrane in order to access substrate through ionic interactions between the phosphate groups of PIP2 and charged residues in the PH domain. While the catalytic core does possess a weak affinity for PIP2, the C2 domain has been shown to mediate calcium-dependent phospholipid binding as well. In this model, the PH and C2 domains operate in concert as a "tether and fix" apparatus necessary for processive catalysis by the enzyme.Leucine
Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...
-rich nuclear export signal
Nuclear export signal
A nuclear export signal is a short amino acid sequence of 4 hydrophobic residues in a protein that targets it for export from the cell nucleus to the cytoplasm through the nuclear pore complex using nuclear transport. It has the opposite effect of a nuclear localization signal, which targets a...
(NES) in its EF hand
EF hand
The EF hand is a helix-loop-helix structural domain found in a large family of calcium-binding proteins. The EF-hand motif contains a helix-loop-helix topology, much like the spread thumb and forefinger of the human hand, in which the Ca2+ ions are coordinated by ligands within the loop...
motif, as well as a Nuclear localization signal
Nuclear localization signal
A nuclear localization signal or sequence is an amino acid sequence which 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines or arginines exposed on the protein surface. Different...
within its linker region. These two elements combined allow PLC-δ1 to actively translocate into and out of the nucleus. However, its function in the nucleus remains unclear.
The widely-expressed PLC-δ1 isoform is the best-characterized phospholipase family member, as it was the first to have high-resolution X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
crystal structures available for analysis. In terms of domain architecture, all of the enzymes are built upon a common PLC-δ backbone, wherein each family displays similarities, as well as obvious distinctions, that contribute to unique regulatory properties within the cell.Because it is the only family found expressed in lower eukaryotic organisms such as yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
and slime molds, it is considered the prototypical PLC isoform. The other family members more than likely evolved from PLC-δ as their domain architecture and mechanism of activation were expanded. Although a full crystal structure has not been obtained, high-resolution X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
has yielded the molecular structure of the N-terminal PH domain complexed with its product IP3, as well as the remainder of the enzyme with the PH domain ablated. These structures have provided researchers with the necessary information to begin speculating about other family members such as PLCβ2.
Other PLC families
- PLC-ε (230-260kDa ) is activated by Ras and RhoRhoRho is the 17th letter of the Greek alphabet. In the system of Greek numerals, it has a value of 100. It is derived from Semitic resh "head"...
GTPases. - PLC-ζ (75kDa) is thought to play an important role in vertebrate fertilization by producing intracellular calcium oscillations important for the start of embryonic development. However, the mechanism of activation still remains unclear. This isoform is also capable of entering the early-formed pronucleusPronucleusA pronucleus is the nucleus of a sperm or an egg cell during the process of fertilization, after the sperm enters the ovum, but before they fuse. Sperm and egg cells are haploid, meaning they carry half the number of chromosomes...
after fertilization, which seems to coincide with the cessation of calcium mobilization. It, like PLC-δ1 and PLC-β, possesses nuclear export and localization sequences. - PLC-η has been implicated in neuronal functioning.
Human proteins in this family
PLCB1PLCB1
1-Phosphatidylinositol-4,5-bisphosphate phosphodiesterase beta-1 is an enzyme that in humans is encoded by the PLCB1 gene.-Further reading:...
; PLCB2
PLCB2
1-Phosphatidylinositol-4,5-bisphosphate phosphodiesterase beta-2 is an enzyme that in humans is encoded by the PLCB2 gene. The gene codes for the enzyme phospholipase C β2. The enzyme catalyzes the formation of inositol 1,4,5-trisphosphate and diacylglycerol from phosphatidylinositol 4,5-bisphosphate...
; PLCB3
PLCB3
1-Phosphatidylinositol-4,5-bisphosphate phosphodiesterase beta-3 is an enzyme that in humans is encoded by the PLCB3 gene. The gene codes for the enzyme phospholipase C β3. The enzyme catalyzes the formation of inositol 1,4,5-trisphosphate and diacylglycerol from phosphatidylinositol 4,5-bisphosphate...
; PLCB4
PLCB4
1-Phosphatidylinositol-4,5-bisphosphate phosphodiesterase beta-4 is an enzyme that in humans is encoded by the PLCB4 gene.- Function :The protein encoded by this gene catalyzes the formation of inositol 1,4,5-trisphosphate and diacylglycerol from phosphatidylinositol 4,5-bisphosphate...
; PLCD1
PLCD1
1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase delta-1 is an enzyme that in humans is encoded by the PLCD1 gene.PLCd1 is essential to maintain homeostasis of the skin.-External links:*...
; PLCD3
PLCD3
1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase delta-3 is an enzyme that in humans is encoded by the PLCD3 gene.A lot of plcd3 express to the brain.- Function :...
; PLCD4
PLCD4
1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase delta-4 is an enzyme that in humans is encoded by the PLCD4 gene....
; PLCE1
PLCE1
1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase epsilon-1 is an enzyme that in humans is encoded by the PLCE1 gene.- Function :...
;
PLCG1
PLCG1
Phospholipase C, gamma 1, also known as PLCG1, is a protein that in humans is encoded by the PLCG1 gene.-Function:The protein encoded by this gene catalyzes the formation of inositol 1,4,5-trisphosphate and diacylglycerol from phosphatidylinositol 4,5-bisphosphate...
; PLCG2
PLCG2
1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase gamma-2 is an enzyme that in humans is encoded by the PLCG2 gene.- Function :Enzymes of the phospholipase C family catalyze the hydrolysis of phospholipids to yield diacylglycerols and water-soluble phosphorylated derivatives of the lipid...
; PLCH1; PLCH2; PLCL1; PLCL2; PLCZ1
See also
- PhospholipasePhospholipaseA phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C and D, distinguished by the type of reaction which they catalyze:*Phospholipase A...
- Alpha toxinAlpha toxinAlpha Toxin or alpha-toxin refers to several different protein toxins produced by bacteria. Alpha toxin may be:*Staphylococcus aureus alpha toxin, a membrane-disrupting toxin that creates pores causing hemolysis and tissue damage....
- Lipid signalingLipid signalingLipid signaling, broadly defined, refers to any biological signaling event involving a lipid messenger that binds a protein target, such as a receptor, kinase or phosphatase, which in turn mediate the effects of these lipids on specific cellular responses...
- Mammalian and bacterial phospholipases - PH domain of phospholipase C delta 1 - α-toxin (a bacterial phospholipase C)