Major Sperm Protein - Structure

Structure

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:

1. 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.
2. 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.
3. fibers, 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, 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.