This domain family is found in eukaryotes, and is approximately 50 amino acids in length. These proteins are found in nematodes. They complex with MSP (major sperm protein) to allow motility. Their action is quite similar to the action of bacterial actin molecules.

1: Structure of MFP2 and its function in enhancing MSP polymerization in Ascaris sperm amoeboid motility. Grant RP, Buttery SM, Ekman GC, Roberts TM, Stewart M; J Mol Biol. 2005;347:583-595. PMID:15755452