Major sperm protein (MSP) is a

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

specific small

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

of 126

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

with a

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

of 14

kDa The dalton or unified atomic mass unit (symbols: Da or u, respectively) is a unit of mass defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. It is a non-SI unit accepted f ...

. It is the key player in the motility machinery of nematodes that propels the

crawling Crawl, The Crawl, or crawling may refer to: Biology * Crawling, any type of tetrapod quadrupedal locomotion with the torso persistently touching or very close to the ground. ** Crawling (human), any of several types of human quadrupedal gait * ...

movement/motility of nematode sperm. It is the most abundant protein present in nematode

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), 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 ...

, 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. There are two forms of reproduction: Asexual reproduction, asexual and Sexual ...

of the

helminthes Parasitic worms, also known as helminths, are a polyphyletic group of 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 para ...

: 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 Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

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 ger ...

maturation and stimulates the

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

wall to contract to bring the oocytes into position for

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

. 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 Hybrid word, blend of the Greek ''caeno-'' (recent), ''r ...

''.

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 ho ...

'' and ''

'' have been determined by

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

and

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic f ...

. 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. Align ...

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 gene ...

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

side-chains from adjacent faces in the sandwich form the interior of the protein. The overall structure of MSP resembles an

immunoglobulin fold The immunoglobulin domain, also known as the immunoglobulin fold, is a type of protein domain that consists of a 2-layer sandwich of 7-9 antiparallel β-strands arranged in two β-sheets with a Greek key topology, consisting of about 125 amino ...

(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 A monomer ( ; ''wikt:mono-, mono-'', "one" + ''wikt:-mer, -mer'', "part") is a molecule that can chemical reaction, react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called ...

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 ** TH-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * ...

is very stable, with putative

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

hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

and

salt bridge In electrochemistry, a salt bridge or ion bridge is an essential laboratory device discovered over 100 years ago. It contains an electrolyte solution, typically an inert solution, used to connect the Redox, oxidation and reduction Half cell, ...

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. An extensive hydrogen bond network is present at the interface between successive dimers along each helix. Residues from one molecule interact with the same residues from the neighboring molecule, forming an antiparallel strand-strand interaction within the β-sheet that connects chain G to chain F. This type of interaction is commonly observed in proteins with an immunoglobulin (Ig) fold. Additionally, the internal pH gradient plays a crucial role in maintaining cytoskeletal structure and locomotion. When the internal pH drops to 5.2, depolymerization occurs, leading to the cessation of cytoskeletal movement. This suggests that a specific ionizable group may be essential for maintaining male sperm protein (MSP) polymerization in vivo. For example, Asp112 is positioned to form hydrogen bonds with both the main chain and side chain nitrogens of Asn120 from the neighboring dimer. Asp112 contributes to four of the ten predicted hydrogen bonds at the interface between MSP dimers. Partial neutralization of this group significantly reduces its hydrogen-bonding potential, which may explain why MSP depolymerizes when the internal cellular pH decreases. # 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 (spelled fibre in British English; from ) 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 inco ...

, 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 ...

, 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 or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

. 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 (: 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 filaments and ...

. 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 cytosk ...

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 Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...

. 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 or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

. 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 -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...

, 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 testicle. This process starts with the Mitosis, mitotic division of the stem cells located close to the basement membrane of ...

by a pH gradient within the

of the

spermatozoon A spermatozoon (; also spelled spermatozoön; : spermatozoa; ) is a motile sperm cell (biology), cell produced by male animals relying on internal fertilization. A spermatozoon is a moving form of the ploidy, haploid cell (biology), cell that is ...

: 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 Substrata, plural of substratum, may refer to: *Earth's substrata, the geologic layering of the Earth *''Hypokeimenon'', sometimes translated as ''substratum'', a concept in metaphysics *Substrata (album), a 1997 ambient music album by Biosphere * ...

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'' ( ...

'',

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 ger ...

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, alig ...

meiosis I Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one co ...

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 serine/threonine-specific protein kinases involved in directing cellular responses to a diverse array of stimuli, such as mitogens, osmotic stress, heat shock and proinflamm ...

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 an important part of the menstrual cycle in female vertebrates where the egg cells are released from the ovaries as part of the ovarian cycle. In female humans ovulation typically occurs near the midpoint in the menstrual cycle and ...

by inducing the enveloping of the

by the

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

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 Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

release, which binds to the v-

SNARE SNARE proteins – "Soluble NSF attachment protein, SNAP REceptors" – are a large protein family consisting of at least 24 members in yeasts and more than 60 members in mammalian and plant cells. The primary role of SNARE proteins is to m ...

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. Synaptobre ...

/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 ...

. 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 amin ...

of the bacterial P-

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

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 b ...

PapD share a high structural and topological homology in their β sheet regions. Both MSP and PapD can be classified to the s-type

proteins, characterized by the above-mentioned unique strand switching.

References

{{Cytoskeletal Proteins Cytoskeleton