MSP domain protein-1 from Ascaris suum and its possible role in the regulation of major sperm protein-based crawling motility

Nematode sperm utilize a crawling motility based on a nematode sperm-specific cytoskeletal protein called the major sperm protein (MSP). Although MSP has no similarity to actin in sequence or structure, the motility mediated by these two proteins is nearly indistinguishable at a phenotypic level. As with the traditional actin cytoskeleton, the central component of MSP-based motility (MSP) interacts with accessory proteins that regulate polymerization and depolymerization and play a key role in cell motility. A bioinformatics approach has led to the identification of proteins with enhanced expression in the Ascaris suum male germ line, including five new proteins each containing an MSP domain. One of these MSP domain proteins (As-MDP-1) contains an MSP domain in the C-terminus and a N-terminal extension rich in prolines and alanines. The 15.6 kDa As-MDP-1 was shown to be >90% identical at the amino acid level to members of a small family of Ascaris proteins that have been shown to bind to the MSP cytoskeleton and to negatively regulate MSP fiber growth. Further, it was demonstrated that As-MDP-1 is the smallest member of the MFP1 triplet of negative regulators of MSP cytoskeleton formation. Antibodies were used to detect the presence of As-MDP-1 along the entire length of the MSP fibers suggesting that As-MDP-1 binds directly to the higher order forms of MSP. This protein has orthologues in other nematode species, is present in Ascaris in at least six allelic forms, and is likely to form multimers.