Amoeboid Cell Motility Research Articles

The Motile Major Sperm Protein (MSP) of Ascaris suum Forms Filaments Constructed from Two Helical Subfilaments

The amoeboid motility nematode sperm is mediated by cytoskeletal filaments composed of major sperm protein (MSP). We have used electron microscopy and image processing to show that MSP filaments are constructed from two subfilament strands which are themselves formed from a helical arrangement of subunits. The subfilaments are based on left-handed helices of pitch 9 nm that then coil along right-handed helical tricks of pitch 22.5 nm to form filaments. The subfilaments appear to be indistinguishable from the helices present in orthorhombic crystals of MSP. Because in filaments the subfilaments are themselves helical, not all subunits are able to participate in protein-protein interactions between different strands. One consequence of this interaction geometry is that the same molecular interactions that function to assemble subfilaments into filaments can also be used to assemble filaments into larger supramolecular assemblies such as the macrofibres formed in vitro and the fibre bundles found in vivo in sperm pseudopods. These results indicate the importance of filament bundling in addition to vectorial filament assembly in amoeboid cell motility.

Supramolecular assemblies of the Ascaris suum major sperm protein (MSP) associated with amoeboid cell motility

Sperm of the nematode, Ascaris suum, are amoeboid cells that do not require actin or myosin to crawl over solid substrata. In these cells, the role usually played by actin has been taken over by major sperm protein (MSP), which assembles into filaments that pack the sperm pseudopod. These MSP filaments are organized into multi-filament arrays called fiber complexes that flow centripetally from the leading edge of the pseudopod to the cell body in a pattern that is intimately associated with motility. We have characterized structurally a hierarchy of helical assemblies formed by MSP. The basic unit of the MSP cytoskeleton is a filament formed by two subfilaments coiled around one another along right-handed helical tracks. In vitro, higher-order assemblies (macrofibers) are formed by MSP filaments that coil around one another in a left-handed helical sense. The multi-filament assemblies formed by MSP in vitro are strikingly similar to the fiber complexes that characterize the sperm cytoskeleton. Thus, self-association is an intrinsic property of MSP filaments that distinguishes these fibers from actin filaments. The results obtained with MSP help clarify the roles of different aspects of the actin cytoskeleton in the generation of locomotion and, in particular, emphasize the contributions made by vectorial assembly and filament bundling.