Nematode sperm are unusual amoeboid cells in which motility is not based on actin, but instead on the major sperm protein (MSP) that polymerizes to form a cytoskeleton constructed from intermeshed filaments. MSP is a dimeric molecule that polymerizes 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 (~14 kDa) 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 the following proteins:

• Animal Vesicle-Associated Membrane Protein-associated (VAMP-associated) protein family of 33 kDa (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 α-helical coiled-coil combined with an extremly 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 β sandwich measuring approximately 15 A x 20 A x 45 A and having opposing three-stranded and four-stranded β sheets (see <PDB:1MSP>) [1].

The profile we developed covers the entire MSP domain.