InterPro domain: IPR000535

Nematode sperm are unusual amoeboid cells in which motility is not based on actin, but instead on the major sperm protein (MSP). MSP is a dimeric molecule that polymerises to form non-polar filaments constructed from two helical subfilaments that wind round one another. The filaments then assemble into larger macromolecular assemblies called fibre complexes. MSP is a small (~14kDa) basic protein typically encoded by a multigene family of up to 28 members [ 1 , 2 , 3 , 4 ]. An about 120-amino acid domain similar to MSP has been found in other proteins, including:

• Animal Vesicle-Associated Membrane Protein-associated (VAMP-associated) protein family of 33kDa (VAP33). VAP33 is required for neurotransmitter release. It binds to the v-SNARE synaptobrevin/VAMP which is associated with vesicle fusion. VAP33 has a two-domain structure with its N terminus being highly homologous to MSP, whereas its C terminus is based on a putative alpha-helical coiled-coil combined with an extremely hydrophobic membrane-attachment region [ 5 ].
• Nicotiana plumbaginifolia VAP27, a VAP33 homologue. It interacts with the resistance protein Cf9 [ 6 ].
• Yeast inositol regulator SCS2, a VAP33 homologue. It is C-terminally anchored to the endoplasmic reticulum [ 7 ].

The MSP polypeptide chain has an immunoglobulin-like fold based on a seven-stranded beta sandwich measuring approximately 15 A x 20 A x 45 A and having opposing three-stranded and four-stranded beta sheets [ 8 ].

This entry represents the MSP domain.

1. 2.5 A resolution crystal structure of the motile major sperm protein (MSP) of Ascaris suum. J. Mol. Biol. 263, 284-96
2. 2.6 A resolution crystal structure of helices of the motile major sperm protein (MSP) of Caenorhabditis elegans. J. Mol. Biol. 319, 491-9
3. Solution structure of the motile major sperm protein (MSP) of Ascaris suum - evidence for two manganese binding sites and the possible role of divalent cations in filament formation. J. Mol. Biol. 284, 1611-24
4. Identification of protein-protein interactions of the major sperm protein (MSP) of Caenorhabditis elegans. J. Mol. Biol. 279, 605-19
5. Molecular cloning and characterization of mammalian homologues of vesicle-associated membrane protein-associated (VAMP-associated) proteins. Biochem. Biophys. Res. Commun. 254, 21-6
6. Molecular cloning and partial characterization of a plant VAP33 homologue with a major sperm protein domain. Biochem. Biophys. Res. Commun. 270, 286-92
7. The Saccharomyces cerevisiae SCS2 gene product, a homolog of a synaptobrevin-associated protein, is an integral membrane protein of the endoplasmic reticulum and is required for inositol metabolism. J. Bacteriol. 180, 1700-8