Snf3
Encyclopedia
Snf3 is a protein which regulates glucose uptake
in yeast. It senses glucose in the environment with high affinity.
sensing and signaling in budding yeast
is similar to the mammalian system
in many ways. However there are also significant differences. Mammalian cells regulate their glucose uptake via hormones (i.e. insulin
and glucagon
) or intermediary metabolites. In contrast, yeast as a unicellular organism does not depend on hormones but on nutrients in the medium. The presence of glucose induces a conformational change
in the membrane proteins Snf3/Rgt2 or Gpr1, and regulates expression of genes involved in glucose metabolism.
to multiple sugar transporters, it shares high similarity to the glucose transporters of rat brain cells and human HepG2 hepatoma cells, as well as to the arabinose
and xylose
transporters (AraE and XylE) of Escherichia coli
. . Based on this homology and on genetic studies, Snf3 was initially thought to be a high affinity glucose transporter.
Later, it was found that Snf3 is not a glucose transporter, but rather a high affinity glucose
sensor. It senses glucose at low concentrations and regulates transcription
of the HXT genes, which encode for glucose transporters. If glucose is absent Snf3 is quiescent and transcription
of the HXT genes is inhibited
by a repressing complex. The complex consisting of several subunits such as Rgt1, Mth1/Std1, Cyc8 and Tup1 binds to the promoters of the HXT genes, thereby blocking their transcription.
Snf3 is able to bind even low amounts of glucose due to its high affinity. The induction of Snf3 by glucose leads to the activation of YckI, a yeast casein
kinase
. This is followed by the recruitment of Mth1 and Std1 to the C-terminus of Snf3 which facilitates the phosphorylation
of the two proteins by YckI. Phosphorylated Mth1 and Std1 are subsequently tagged for proteasome
dependent degradation by SCFGrrl
, an E3 ubiquitin ligase. Therefore, the inhibitory complex misses two of its key components and cannot be assembled. Thus, repression of the HXT genes is abolished, leading to the expression of the glucose transporters and subsequently glucose import.
in yeasts that consists of 12 (2x6) transmembrane domains, like the homologous glucose transporters. Its structure is distinct from the homologous transporters in particular by a long C-terminal tail which is predicted to reside in the cytoplasm
.
The long C-terminal tail plays an important role in glucose signaling and is probably the signaling domain itself. A soluble version of the C-terminal tail alone is sufficient to induce glucose transport.
All glucose transporters including Snf3 contain an arginine
residue situated in a cytoplasmic loop preceding the fifth transmembrane domain. If this position is mutated, Snf3 adopts a state of constant glucose induction irrespective of whether there are nutrients present or not; this suggests an involvement in the glucose sensing process.
. Snf3 and Rgt2 influence directly or indirectly several Hxt-transporters which are responsible for the glucose uptake. Low extracellular
glucose concentrations are sensed by the Snf3 protein which probably leads to the expression of Hxt2-Genes for high affinity glucose transporters, while Rgt2 senses high glucose concentrations and leads to the expression of low affinity glucose transporters, like Hxt1 Although the downstream pathway is poorly understood it seems that Snf3 and Rgt2 transmit a signal directly or indirectly to Grr1, the DNA binding protein Rgt1, and the two cofactors Ssn6 and Tup1. Also needed for the transcription are the two nuclear proteins Mth1 and Std1.
Glucose transporter
Glucose transporters are a wide group of membrane proteins that facilitate the transport of glucose over a plasma membrane. Because glucose is a vital source of energy for all life these transporters are present in all phyla...
in yeast. It senses glucose in the environment with high affinity.
Introduction
GlucoseGlucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
sensing and signaling in budding yeast
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...
is similar to the mammalian system
Glucose uptake
Method of glucose uptake differs throughout tissues depending on two factors; the metabolic needs of the tissue and availability of glucose. The two ways in which glucose uptake can take place are facilitated diffusion and secondary active transport .- Facilitated diffusion :There are over 10...
in many ways. However there are also significant differences. Mammalian cells regulate their glucose uptake via hormones (i.e. insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
and glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
) or intermediary metabolites. In contrast, yeast as a unicellular organism does not depend on hormones but on nutrients in the medium. The presence of glucose induces a conformational change
Conformational change
A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change...
in the membrane proteins Snf3/Rgt2 or Gpr1, and regulates expression of genes involved in glucose metabolism.
Homology and function
Snf3 is homologousHomology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
to multiple sugar transporters, it shares high similarity to the glucose transporters of rat brain cells and human HepG2 hepatoma cells, as well as to the arabinose
Arabinose
Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde functional group.For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, or structurally analogous to D-glyceraldehyde.For sugars, the D/L...
and xylose
Xylose
Xylose is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass...
transporters (AraE and XylE) of Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
. . Based on this homology and on genetic studies, Snf3 was initially thought to be a high affinity glucose transporter.
Later, it was found that Snf3 is not a glucose transporter, but rather a high affinity glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
sensor. It senses glucose at low concentrations and regulates transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
of the HXT genes, which encode for glucose transporters. If glucose is absent Snf3 is quiescent and transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
of the HXT genes is inhibited
Transcriptional regulation
Transcriptional regulation is the change in gene expression levels by altering transcription rates. -Regulation of transcription:Regulation of transcription controls when transcription occurs and how much RNA is created...
by a repressing complex. The complex consisting of several subunits such as Rgt1, Mth1/Std1, Cyc8 and Tup1 binds to the promoters of the HXT genes, thereby blocking their transcription.
Snf3 is able to bind even low amounts of glucose due to its high affinity. The induction of Snf3 by glucose leads to the activation of YckI, a yeast casein
Casein
Casein is the name for a family of related phosphoprotein proteins . These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 60% and 65% of the proteins in human milk....
kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
. This is followed by the recruitment of Mth1 and Std1 to the C-terminus of Snf3 which facilitates the 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 the two proteins by YckI. Phosphorylated Mth1 and Std1 are subsequently tagged for proteasome
Proteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
dependent degradation by SCFGrrl
SCF complex
Skp, Cullin, F-box containing complex is a multi-protein E3 ubiquitin ligase complex catalyzing the ubiquitination of proteins destined for proteasomal degradation...
, an E3 ubiquitin ligase. Therefore, the inhibitory complex misses two of its key components and cannot be assembled. Thus, repression of the HXT genes is abolished, leading to the expression of the glucose transporters and subsequently glucose import.
Structure
Snf3 is a plasma membrane proteinMembrane protein
A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes.-Function:...
in yeasts that consists of 12 (2x6) transmembrane domains, like the homologous glucose transporters. Its structure is distinct from the homologous transporters in particular by a long C-terminal tail which is predicted to reside in 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...
.
The long C-terminal tail plays an important role in glucose signaling and is probably the signaling domain itself. A soluble version of the C-terminal tail alone is sufficient to induce glucose transport.
All glucose transporters including Snf3 contain an arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
residue situated in a cytoplasmic loop preceding the fifth transmembrane domain. If this position is mutated, Snf3 adopts a state of constant glucose induction irrespective of whether there are nutrients present or not; this suggests an involvement in the glucose sensing process.
Regulation
The regulation of Snf3 in S. cerevisiae and its downstream events are still poorly understood, but it seems clear that a second glucose sensor Rgt2 influences Snf3 and vice versa. Furthermore, it is unclear whether these two proteins sense the glucose concentration on the outside or inside the cellCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. Snf3 and Rgt2 influence directly or indirectly several Hxt-transporters which are responsible for the glucose uptake. Low extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...
glucose concentrations are sensed by the Snf3 protein which probably leads to the expression of Hxt2-Genes for high affinity glucose transporters, while Rgt2 senses high glucose concentrations and leads to the expression of low affinity glucose transporters, like Hxt1 Although the downstream pathway is poorly understood it seems that Snf3 and Rgt2 transmit a signal directly or indirectly to Grr1, the DNA binding protein Rgt1, and the two cofactors Ssn6 and Tup1. Also needed for the transcription are the two nuclear proteins Mth1 and Std1.