Major sperm protein

The major sperm protein (MSP) is a nematode specific small protein of 126 amino acids with a molecular weight of 14 kDa. 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, comprising 15% of the total protein and more than 40% of the soluble protein. MSP is exclusively synthesized in spermatocytes of the nematodes. The MSP has two main functions in the reproduction of the helminthes: 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 on the female germ cells, where it triggers oocyte maturation and stimulates the oviduct wall to contract to bring the oocytes into position for fertilization. MSP has first been identified in Caenorhabditis elegans. The major sperm protein (MSP) is a nematode specific small protein of 126 amino acids with a molecular weight of 14 kDa. 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, comprising 15% of the total protein and more than 40% of the soluble protein. MSP is exclusively synthesized in spermatocytes of the nematodes. The MSP has two main functions in the reproduction of the helminthes: 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 on the female germ cells, where it triggers oocyte maturation and stimulates the oviduct wall to contract to bring the oocytes into position for fertilization. MSP has first been identified in Caenorhabditis elegans. Molecular structures of MSP from Ascaris suum and Caenorhabditis elegans have been determined by X-ray crystallography and NMR spectroscopy. MSP molecules from these species share 83% sequence identity 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. Hydrophobic 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 a dimer is very stable, with putative hydrophobic, hydrogen bond and salt bridge 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: In contrast to actin, 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. It was suggested that ATP activates either membrane-bound MSP filament end-tracking proteins or their soluble cofactors. Nematode sperm move in an amoeboidal manner by extending a pseudopod. Unlike the motility of actin-based cells, which is based on polar cytoskeletal elements such as actin monomers or tubulin dimers, nematode sperm locomotion is based on a pseudopod and a cytoskeleton built out of a meshwork of non-polar MSP filaments. The two main differences between actin 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. Locomotion in nematodes occurs by localized extension of the leading edge of the pseudopod, attachment of the cytoskeleton to the substrate, and retraction of the cell. Assembly of MSP filaments at the leading edge together with disassembly at the base of the pseudopod results in a treadmilling motion, which corresponds to the crawling locomotion of nematode sperm.