Major sperm protein (MSP) is a

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...

specific small

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

of 126

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

with a

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

of 14

kDa The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at re ...

. It is the key player in the motility machinery of nematodes that propels the crawling movement/motility of nematode sperm. It is the most abundant protein present in nematode

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

, comprising 15% of the total protein and more than 40% of the soluble protein. MSP is exclusively synthesized in

spermatocytes Spermatocytes are a type of male gametocyte in animals. They derive from immature germ cells called spermatogonia. They are found in the testis, in a structure known as the seminiferous tubules. There are two types of spermatocytes, primary and s ...

of the nematodes. The MSP has two main functions in the

reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...

of the

helminthes Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as schi ...

: i) as cytosolic component it is responsible for the crawling movement of the mature sperm (without flagellum), and ii) once released, it acts as

hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...

on the female germ cells, where it triggers

oocyte An oocyte (, ), oöcyte, or ovocyte 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 a female fetus in the ovary during female gametogenesis. The female ...

maturation and stimulates the

oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...

wall to contract to bring the oocytes into position for

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

. MSP has first been identified in ''

Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (ro ...

''.

Structure

Molecular structures Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that deter ...

of MSP from ''

Ascaris suum ''Ascaris suum'', also known as the large roundworm of pig, is a parasitic nematode that causes ascariasis in pigs. While roundworms in pigs and humans are today considered as two species (''A. suum'' and '' A. lumbricoides'') with different hos ...

'' and ''

'' have been determined by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

and

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...

. MSP molecules from these species share 83%

sequence identity In bioinformatics, a sequence alignment is a way of arranging the sequences of DNA, RNA, or protein to identify regions of similarity that may be a consequence of functional, structural, or evolutionary relationships between the sequences. Ali ...

and their structures are highly similar. MSP does not harbor any known conserved domain. It is made of a seven-stranded β sandwich, having opposing three-stranded and four-stranded

β sheets The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a g ...

Hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

side-chains from adjacent faces in the sandwich form the interior of the protein. The overall structure of MSP resembles an immunoglobulin fold (Ig fold). MSP can be classified as an s-type of this fold, because two of its strands are switching between separate β sheets, unlike in the conserved c-type of the Ig folds. The unique strand switches between the sheets result from two distinct kinks at ''cis''-proline residues 13 and 57 in ''A. suum'' protein. MSP monomers form symmetric dimers. The interaction between MSP

monomers In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification M ...

in a

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ...

is very stable, with putative

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

and

salt bridge In electrochemistry, a salt bridge or ion bridge is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell (voltaic cell), a type of electrochemical cell. It maintains electrical neutrality within the in ...

interactions. The residues involved in interface formation are between residue 13 and 29 in both ''A. suum'' MSP chains of the dimer. MSP spontaneously polymerises both ''in vivo'' and ''in vitro'' from dimers into subfilaments, filaments, larger bundles and filament networks. MSP dimers are the smallest building blocks for these assemblies, none of which have overall polarity: #subfilaments, formed from dimers, connected to a long helix. The dimer-dimer interface within the single subfilament is formed by residues 112-119 of two ''A. suum'' MSP chains, which produce an anti-parallel β-strand-β-strand pairing. The interaction is less hydrophobic and results mostly from formation of hydrogen bonds, typically for interfaces between reversibly interacting molecules. # filaments, formed by two subfilaments coiling round one another. The MSP dimer-dimer interactions between two adjacent subfilaments in the filament are characterized by five interfaces, mostly between the residues 78-85 and 98-103. Amino acids 78-85 are part of a highly exposed surface loop connecting different β sheets and are divergent between ''C. elegans'' and ''A. suum''. However, the loop consisting of 98-103 residues is highly conserved between all isoforms in both species of the nematode. #

fibers Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...

, macrofibers or bundles, produced by supercoiling of the filaments. ''A. suum'' MSP filaments frequently form rope-like structures called macrofibers. ''C. elegans'' MSP mostly form rafts in which a number of filaments are arranged parallel to one another. In contrast to

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...

, MSP lacks an ATP-binding site. However, it was noticed that ATP is required for MSP filament assembly at the surface of the

plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...

. It was suggested that ATP activates either membrane-bound MSP filament end-tracking proteins or their soluble

cofactors Cofactor may also refer to: * Cofactor (biochemistry), a substance that needs to be present in addition to an enzyme for a certain reaction to be catalysed * A domain parameter in elliptic curve cryptography, defined as the ratio between the order ...

Functions

Sperm motility

Nematode sperm move in an amoeboidal manner by extending a

pseudopod A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filamen ...

. Unlike the motility of actin-based cells, which is based on polar cytoskeletal elements such as

monomers or

tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...

dimers, nematode sperm locomotion is based on a

and a cytoskeleton built out of a meshwork of non-polar MSP filaments. The two main differences between

and MSP is that MSP does not bind ATP and the lack of polarity in MSP, thus disabling motility through motor proteins, such as

myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility. The first myosin ...

. Locomotion in nematodes occurs by localized extension of the leading edge of the

, attachment of the

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...

to the substrate, and retraction of the cell. Assembly of MSP filaments at the leading edge together with disassembly at the base of the

results in a treadmilling motion, which corresponds to the crawling locomotion of nematode sperm. Nematode sperm motility is based on a push-pull mechanism which requires two forces triggered by a pH gradient along the

: one protrusive force and another traction force. The protrusive force is located at the leading edge and pushes against the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

. This force is generated by polymerization of the MSP filaments. MSP filaments are assembled in the cytoplasm near the leading edge of the

out of MSP dimers resulting in extensions. These extensions allow the interaction of the filament complexes with surrounding complexes, thus resulting in an interconnected uniform

and leading to the crawling movement of the

. MSP filament assembly is triggered by external factors, such as changes in pH, the integral membrane phosphoprotein (MPOP), and MSP domain proteins (MDPs). A 48 kDa integral membrane

phosphoprotein A phosphoprotein is a protein that is posttranslationally modified by the attachment of either a single phosphate group, or a complex molecule such as 5'-phospho-DNA, through a phosphate group. The target amino acid is most often serine, threonin ...

, the major sperm protein polymerization organization protein (MPOP), is the starting point of the

and is required for the localized membrane-associated polymerization of MSP. This protein is distributed in vesicles throughout the pseudopodal membrane.

Tyrosine kinases A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to the tyrosine residues of specific proteins inside a cell. It functions as an "on" or "off" switch in many cellular functions. Tyrosine kinases belong to a larger cla ...

, which are pH sensitive, phosphorylate the tyrosine residues of MPOP localized at the tip of the pseudopod, thus resulting in the polymerization of the MSP filaments. In ''Ascaris suum'', two MSP-fiber proteins (MFP), MFP1 and 2, with opposite effect on polymerization have been identified. MFP1 inhibits and MFP2 stimulates the MSP assembly. Changes in pH both controls and activates the MSP polymerization throughout

spermatogenesis Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubule ...

by a pH gradient within the

of the

spermatozoon A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, t ...

: assembly occurs at the leading edge where the pH is high, and disassembly of the filaments occurs at the base where the pH is lowered. Degradation of the MSP filaments results in a traction force at the base of the

, which in turn pulls the

forward. The combination of these two forces is the motive force that allows sperm motility. Attachment of the cytoskeleton to the

substratum In linguistics, a stratum (Latin for "layer") or strate is a language that influences or is influenced by another through contact. A substratum or substrate is a language that has lower power or prestige than another, while a superstratum or sup ...

is required to generate a directional movement.

Affecting female germ cells

MSP affects oocytes on two levels: # MSP regulates oocyte maturation. In ''

C. elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...

'',

oocytes An oocyte (, ), oöcyte, or ovocyte 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 a female fetus in the ovary during female gametogenesis. The female ge ...

arrest their meiotic cycle at

metaphase Metaphase ( and ) is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage (they are at their most condensed in anaphase). These chromosomes, carrying genetic information, align ...

meiosis I Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...

where it is only resumed in presence of sperms. MSP was identified as the molecular factor triggering

meiotic maturation. It is secreted by the sperms through a vesicular budding mechanism and forms an extracellular concentration gradient. MSP binds to VAB-1 which is an

Eph receptor Eph receptors (Ephs, after erythropoietin-producing human hepatocellular receptors) are a group of receptors that are activated in response to binding with Eph receptor-interacting proteins (Ephrins). Ephs form the largest known subfamily of rec ...

protein-tyrosine kinase A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to the tyrosine residues of specific proteins inside a cell. It functions as an "on" or "off" switch in many cellular functions. Tyrosine kinases belong to a larger cla ...

. In absence of MSP, the VAB-1 Eph receptor inhibits meiotic maturation of

through interaction with inhibitors DAB-1/Disabled and RAN-1. MSP binding prevents this inhibition and results in activation of

MAPK A mitogen-activated protein kinase (MAPK or MAP kinase) is a type of protein kinase that is specific to the amino acids serine and threonine (i.e., a serine/threonine-specific protein kinase). MAPKs are involved in directing cellular responses to ...

pathway. # MSP also stimulates gonadal sheath cell contraction which is a

myoepithelial Myoepithelial cells (sometimes referred to as myoepithelium) are cells usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the luminal cells. These may be positive for alpha smooth muscle actin a ...

sheath surrounding proximal

. It increases the contraction rate from 10-13 to around 19 contractions per minute. The importance of these contractions is promoting

ovulation Ovulation is the release of eggs from the ovaries. In women, this event occurs when the ovarian follicles rupture and release the secondary oocyte ovarian cells. After ovulation, during the luteal phase, the egg will be available to be fertilized ...

by inducing the enveloping of the

by the

spermatheca The spermatheca (pronounced plural: spermathecae ), also called receptaculum seminis (plural: receptacula seminis), is an organ of the female reproductive tract in insects, e.g. ants, bees, some molluscs, oligochaeta worms and certain other ...

Homologues

MSP genes have been identified across widely diverged

species. They all have more than 60%

. Proteins with limited sequence similarity were identified in species from plants to mammals. One of the homologues is VAP33 from ''

Aplysia californica The California sea hare (''Aplysia californica'') is a species of sea slug in the sea hare family, Aplysiidae.Rosenberg, G.; Bouchet, P. (2011). Aplysia californica J. G. Cooper, 1863. Accessed through: World Register of Marine Species at http:// ...

''. VAP33 is a protein required for

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neuro ...

release, which binds to the v-

SNARE SNARE proteins – " SNAP REceptor" – are a large protein family consisting of at least 24 members in yeasts, more than 60 members in mammalian cells, and some numbers in plants. The primary role of SNARE proteins is to mediate vesicle fu ...

synaptobrevin Synaptobrevins (''synaptobrevin isotypes 1-2'') are small integral membrane proteins of secretory vesicles with molecular weight of 18 kilodalton (kDa) that are part of the vesicle-associated membrane protein (VAMP) family. Synaptobrevi ...

/VAMP, associated with

vesicle fusion Vesicle fusion is the merging of a vesicle with other vesicles or a part of a cell membrane. In the latter case, it is the end stage of secretion from secretory vesicles, where their contents are expelled from the cell through exocytosis. Vesicles c ...

. Despite only 11% of sequence similarity, MSP and the

N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...

of the bacterial P-

pilus A pilus (Latin for 'hair'; plural: ''pili'') is a hair-like appendage found on the surface of many bacteria and archaea. The terms ''pilus'' and '' fimbria'' (Latin for 'fringe'; plural: ''fimbriae'') can be used interchangeably, although some r ...

associated

chaperonin HSP60, also known as chaperonins (Cpn), is a family of heat shock proteins originally sorted by their 60kDa molecular mass. They prevent misfolding of proteins during stressful situations such as high heat, by assisting protein folding. HSP60 bel ...

PapD share a high structural and topological homology in their β sheet regions. Both MSP and PapD can be classified to the s-type immunoglobulin fold proteins, characterized by the above-mentioned unique strand switching.

References

{{Cytoskeletal Proteins Cytoskeleton