Cysteine-rich secretory protein
Encyclopedia
Cysteine-rich secretory proteins, often abbreviated as CRISPs, are a group of glycoprotein
s found exclusively in vertebrate
s. They are a subgroup of the CRISP, antigen 5 and Pr-1 (CAP
) protein superfamily and are substantially implicated in the functioning of the mammal
ian reproductive system. CRISPs are also found in a variety of snake venom
s where they inhibit both smooth muscle contraction and cyclic nucleotide-gated ion channel
s.
s are conjugated protein
s in which the non-protein group is a carbohydrate
glycan – typically an oligosaccharide
or small polysaccharide
, but occasionally a monosaccharide
. The glycan is covalently bound
to a side chain
of the polypeptide, rather than to the C- or N-terminus of the protein
in a process called glycosylation
. CRISPs are glycoproteins in which the primary structure is rich in the amino acid cysteine
. Cysteine residues are typically oxidised to cystine
in proteins, as the formation of disulfide bond
s plays an important role in protein folding
and the stabilisation of tertiary structure – this is particularly important with proteins secreted
to the extracellular
medium such as CRISPs. However, CRISPs are atypical in that they often possess significant numbers of native (unoxidised) cysteine residues in addition to cystine residues involved in disulfide bonds. The
King Cobra
venom ophanin
, for example, has 16 strictly conserved cysteines in addition to 8 disulfide bonds.
of mammals, and are also involved in the process of fertilisation
. In the spermatogenesis
process (development of the spermatozoa in the testis), the CRISP2
protein
is incorporated into the acrosome
where it is believed to be involved in the adhesion
of germ cell
s with Sertoli cell
s. CRISP2 also forms part of the sperm
tail where it is thought to be involved in regulating flagellar beating
. Proteins CRISP1
and CRISP4 are both found in the epididymis where they are also incorporated within the spermatozoa as it matures. Protein CRISP3
is found in seminal fluid, excreted from the prostate
although its function is presently unknown.
During capacitation
, the penultimate stage of spermatozoa maturation, the acrosomal sperm head membrane is destabilised to allow greater binding between oocyte
and sperm. CRISP1 binds to surface of the sperm leading to a quiescent state of storage prior to capacitation. The mechanism is believed to involve inhibition of ion channel
activity, similar to the mechanism of action of the other major function of CRISPs in snake venom
. Research also suggests that CRISPs are involved in the oocyte-sperm binding needed for fertilisation. Given the involvement of CRISPs in several stages of human reproduction
, it is unsurprising that applications in treatment of infertility
and as contraceptives are being actively investigated.
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CRISPs are found in the venom
of a wide variety of snake
species. Examples include ablomin
from the Japanese Mamushi
snake (Gloydius blomhoffi, formerly Agkistrodon
blomhoffi), latisemin
from the Erabu sea snake (Laticauda semifasciata), ophanin
from the King Cobra (Ophiophagus hannah), piscivorin
from the Eastern Cottonmouth (Agkistrodon piscivorus
) and triflin
from the Habu snake (Trimeresurus flavoviridis
) – each of these proteins is named for the snake species in which it was discovered. These venoms are toxic due to their blocking of calcium channel
s and also because they reduce potassium-induced smooth muscle contraction. Among the four CRISPs isolated from the Monocled Cobra (Naja kaouthia
) and the three from the Egyptian Cobra (Naja haje), ion channel activity occurred by blocking of cyclic nucleotide-gated ion channel
s. One of the N. haje CRISPs was the first example of an acid
ic CRISP in reptilian venom. The selective ion channel activity of snake CRISPs, coupled with the variety of CRISPs available as the pool of venom proteins appears highly variable between (at least) cobra species, provide a valuable tool for probing the mechanisms of ion channel activity.
Glycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
s found exclusively in vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s. They are a subgroup of the CRISP, antigen 5 and Pr-1 (CAP
CAP protein family
In molecular biology, the CAP protein family is a large family of proteins that are found in a wide range of organisms, including prokaryotes and non-vertebrate eukaryotes...
) protein superfamily and are substantially implicated in the functioning of the mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian reproductive system. CRISPs are also found in a variety of snake venom
Snake venom
Snake venom is highly modified saliva that is produced by special glands of certain species of snakes. The glands which secrete the zootoxin are a modification of the parotid salivary gland of other vertebrates, and are usually situated on each side of the head below and behind the eye,...
s where they inhibit both smooth muscle contraction and cyclic nucleotide-gated ion channel
Cyclic nucleotide-gated ion channel
Cyclic nucleotide-gated ion channels are ion channels that function in response to the binding of cyclic nucleotides. CNG channels are nonselective cation channels that are found in the membranes of various types of cells.- Discovery :...
s.
Structure
GlycoproteinGlycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
s are conjugated protein
Conjugated protein
A conjugated protein is a protein that functions in interaction with other chemical groups attached by covalent bonds or by weak interactions.Many proteins contain only amino acids and no other chemical groups, and they are called simple proteins...
s in which the non-protein group is a carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
glycan – typically an oligosaccharide
Oligosaccharide
An oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...
or small polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
, but occasionally a monosaccharide
Monosaccharide
Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...
. The glycan is covalently bound
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
to a side chain
Side chain
In organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called "main chain" or backbone. The placeholder R is often used as a generic placeholder for alkyl group side chains in chemical structure diagrams. To indicate other non-carbon...
of the polypeptide, rather than to the C- or N-terminus of the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
in a process called glycosylation
Glycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
. CRISPs are glycoproteins in which the primary structure is rich in the amino acid cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
. Cysteine residues are typically oxidised to cystine
Cystine
Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...
in proteins, as the formation of disulfide bond
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
s plays an important role in protein folding
Protein folding
Protein folding is the process by which a protein structure assumes its functional shape or conformation. It is the physical process by which a polypeptide folds into its characteristic and functional three-dimensional structure from random coil....
and the stabilisation of tertiary structure – this is particularly important with proteins secreted
Secretory protein
A secretory protein is any protein, whether it be endocrine or exocrine, which is secreted by a cell. Secretory proteins include many hormones, enzymes, toxins, and antimicrobial peptides.Secretory proteins are synthesized in endoplasmic reticulum....
to the 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...
medium such as CRISPs. However, CRISPs are atypical in that they often possess significant numbers of native (unoxidised) cysteine residues in addition to cystine residues involved in disulfide bonds. The
King Cobra
King Cobra
The king cobra is the world's longest venomous snake, with a length up to 5.6 m . This species, which preys chiefly on other snakes, is found predominantly in forests from India through Southeast Asia to the Philippines and Indonesia...
venom ophanin
Ophanin
Ophanin is a toxin found in the venom of the King Cobra , which lives throughout South East Asia. This toxin belongs to the cysteine-rich secretory protein family...
, for example, has 16 strictly conserved cysteines in addition to 8 disulfide bonds.
Mammalian reproduction
CRISPs are found in the testes and epididymisEpididymis
The epididymis is part of the male reproductive system and is present in all male amniotes. It is a narrow, tightly-coiled tube connecting the efferent ducts from the rear of each testicle to its vas deferens. A similar, but probably non-homologous, structure is found in cartilaginous...
of mammals, and are also involved in the process of fertilisation
Fertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
. In the spermatogenesis
Spermatogenesis
Spermatogenesis is the process by which male primary germ cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived. Each primary spermatocyte divides into two secondary spermatocytes, and each secondary spermatocyte into two...
process (development of the spermatozoa in the testis), the CRISP2
CRISP2
Cysteine-rich secretory protein 2 is a cysteine-rich secretory protein that in humans is encoded by the CRISP2 gene.-Further reading:...
protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
is incorporated into the acrosome
Acrosome
The acrosome is an organelle that develops over the anterior half of the head in the spermatozoa of many animals. It is a cap-like structure derived from the Golgi apparatus. Acrosome formation is completed during testicular maturation. In Eutherian mammals the acrosome contains digestive enzymes...
where it is believed to be involved in the adhesion
Cell adhesion
Cellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as selectins, integrins, and cadherins. Correct cellular adhesion is essential in maintaining multicellular structure...
of germ cell
Germ cell
A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate near the gut of an embryo and migrate to the developing gonads. There, they undergo cell division of two types, mitosis and meiosis, followed by...
s with Sertoli cell
Sertoli cell
A Sertoli cell is a 'nurse' cell of the testes that is part of a seminiferous tubule.It is activated by follicle-stimulating hormone and has FSH-receptor on its membranes.-Functions:...
s. CRISP2 also forms part of the sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
tail where it is thought to be involved in regulating flagellar beating
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
. Proteins CRISP1
CRISP1
Cysteine-rich secretory protein 1 is a cysteine-rich secretory protein that in humans is encoded by the CRISP1 gene.-Further reading:...
and CRISP4 are both found in the epididymis where they are also incorporated within the spermatozoa as it matures. Protein CRISP3
CRISP3
Cysteine-rich secretory protein 3 is a cysteine-rich secretory protein that in humans is encoded by the CRISP3 gene.-Further reading:...
is found in seminal fluid, excreted from the prostate
Prostate
The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals....
although its function is presently unknown.
During capacitation
Capacitation
Capacitation is the penultimate step in the maturation of mammalian spermatozoa and is required to render them competent to fertilize an oocyte.This step is a biochemical event; the sperm move normally and look mature prior to capacitation....
, the penultimate stage of spermatozoa maturation, the acrosomal sperm head membrane is destabilised to allow greater binding between oocyte
Oocyte
An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
and sperm. CRISP1 binds to surface of the sperm leading to a quiescent state of storage prior to capacitation. The mechanism is believed to involve inhibition of 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...
activity, similar to the mechanism of action of the other major function of CRISPs in snake venom
Snake venom
Snake venom is highly modified saliva that is produced by special glands of certain species of snakes. The glands which secrete the zootoxin are a modification of the parotid salivary gland of other vertebrates, and are usually situated on each side of the head below and behind the eye,...
. Research also suggests that CRISPs are involved in the oocyte-sperm binding needed for fertilisation. Given the involvement of CRISPs in several stages of human reproduction
Human reproduction
Human reproduction is any form of sexual reproduction resulting in the conception of a child, typically involving sexual intercourse between a man and a woman. During intercourse, the interaction between the male and female reproductive systems results in fertilization of the woman's ovum by the...
, it is unsurprising that applications in treatment of infertility
Infertility
Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...
and as contraceptives are being actively investigated.
Snake venom
Venom
Venom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite or a sting...
of a wide variety of snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
species. Examples include ablomin
Ablomin
Ablomin is a toxin present in the venom of the Japanese Mamushi snake, which blocks L-type voltage-gated calcium channels.- Etymology :The protein ablomin is a component of the venom of the Japanese Mamushi snake, Gloydius blomhoffi...
from the Japanese Mamushi
Mamushi
Gloydius blomhoffii, commonly known as the mamushi, or Japanese mamushi, is a venomous pitviper species found in China, Japan, and Korea. There are four subspecies including the nominate subspecies described here....
snake (Gloydius blomhoffi, formerly Agkistrodon
Agkistrodon
Agkistrodon is a genus of venomous pit vipers found in North America from the United States south to northern Costa Rica. The name is derived from the Greek words ἄγκιστρον 'fishhook' and ὁδοὐς 'tooth', and is likely a reference to the fangs...
blomhoffi), latisemin
Latisemin
Latisemin is a cysteine-rich secretory protein that can be isolated from the venom of the Black-banded sea krait, a sea snake indigineous to the warmer waters of the western Pacific Ocean...
from the Erabu sea snake (Laticauda semifasciata), ophanin
Ophanin
Ophanin is a toxin found in the venom of the King Cobra , which lives throughout South East Asia. This toxin belongs to the cysteine-rich secretory protein family...
from the King Cobra (Ophiophagus hannah), piscivorin
Piscivorin
Piscivorin is a component of snake venom secreted by the Eastern Cottonmouth . It is a member of the cysteine-rich secretory protein family, which blocks voltage-dependent calcium channels.- Etymology :...
from the Eastern Cottonmouth (Agkistrodon piscivorus
Agkistrodon piscivorus
Agkistrodon piscivorus is a venomous snake, a species of pit viper, found in the southeastern United States. Adults are large and capable of delivering a painful and potentially fatal bite. When antagonized they will stand their ground by coiling their bodies and displaying their fangs...
) and triflin
Triflin
Triflin is a cysteine-rich secretory protein , which is excreted by the venom gland of the Habu snake . Triflin reduces high potassium-induced smooth muscle contraction, suggesting a blocking effect on L-type calcium channels.- Source :Triflin is a toxin derived from snake venom...
from the Habu snake (Trimeresurus flavoviridis
Trimeresurus flavoviridis
Trimeresurus flavoviridis is a venomous pitviper species found in Japan in the Ryukyu Islands. No subspecies are currently recognized.-Description:...
) – each of these proteins is named for the snake species in which it was discovered. These venoms are toxic due to their blocking of calcium channel
Calcium channel
A Calcium channel is an ion channel which displays selective permeability to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...
s and also because they reduce potassium-induced smooth muscle contraction. Among the four CRISPs isolated from the Monocled Cobra (Naja kaouthia
Naja kaouthia
The monocled cobra is a cobra species, which is widespread across central and southern Asia. It has been assessed as Least Concern by IUCN owing to its large distribution, tolerance of a broad range of habitats, including anthropogenically altered environments, and its reported abundance...
) and the three from the Egyptian Cobra (Naja haje), ion channel activity occurred by blocking of cyclic nucleotide-gated ion channel
Cyclic nucleotide-gated ion channel
Cyclic nucleotide-gated ion channels are ion channels that function in response to the binding of cyclic nucleotides. CNG channels are nonselective cation channels that are found in the membranes of various types of cells.- Discovery :...
s. One of the N. haje CRISPs was the first example of an acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
ic CRISP in reptilian venom. The selective ion channel activity of snake CRISPs, coupled with the variety of CRISPs available as the pool of venom proteins appears highly variable between (at least) cobra species, provide a valuable tool for probing the mechanisms of ion channel activity.