THE MOTILE MAJOR SPERM PROTEIN (MSP) OF ASCARIS-SUUM FORMS FILAMENTS CONSTRUCTED FROM 2 HELICAL SUBFILAMENTS

The amoeboid motility nematode sperm is mediated by cytoskeletal filam ents composed of major sperm protein (MSP). me have used electron micr oscopy and image processing to show that MSP filaments are constructed from two subfilament strands which are themselves formed from a helic al arrangement of subunits. The subfilaments are based on left-handed helices of pitch 9 nm that then coil along right-handed helical tracks of pitch 22.5 nm to form filaments. The subfilaments appear to be ind istinguishable from the helices present in orthorhombic crystals of Ms P. Because in filaments the subfilaments are themselves helical, not a ll subunits are able to participate in protein-protein interactions be tween different strands. One consequence of this interaction geometry is that the same molecular interactions that function to assemble subf ilaments into filaments can also be used to assemble filaments into la rger supramolecular assemblies such as the macrofibres formed in vitro and the fibre bundles found in vivo in sperm pseudopods. These result s indicate the importance of filament bundling in addition to vectoria l filament assembly in amoeboid cell motility.