Solution structure of the motile major sperm protein (MSP) of Ascaris suum- Evidence for two manganese binding sites and the possible role of bivalent cations in filament formation

The major sperm protein (MSP) of Ascaris suum mediates amoeboid motility by forming an extensive intermeshed system of cytoskeletal filaments analogou s to that formed by actin in many other amoeboid cells. MSP is a dimeric mo lecule that polymerizes to form non-polar filaments constructed from two he lical subfilaments that wind round one another. Moreover, MSP filaments can interact with one another to form higher-order assemblies without requirin g the range of accessory proteins usually employed in actin-based systems. A knowledge of how MSP polymerizes and forms the hierachical series of heli cal MSP macromolecular assemblies is fundamenatal to understanding locomoti on in these cells. Here we describe the solution structure of MSP dimers de termined by NMR spectroscopy under conditions where MSP does not polymerize to form filaments. The solution structure is indistinguishable from that o bserved in putative MSP subfilament helices by X-ray crystallography, indic ating that MSP polymerization is not accompanied by a major conformational change. We also show that the rate of MSP polymerization associated with mo vement of vesicles in an in vitro motility assay is enhanced by the presenc e of magnesium and manganese ions and use NMR to show that the primary resi dues that bind these ions are 24-25 and 83-86. These residues are distant f rom the interface formed between MSP dimers in subfilament helices, and so are probably not involved in this level of polymerization. Instead the mang anese and magnesium ion binding appears to be associated with the assembly of subfilaments into filaments and their subsequent aggregation into bundle s. (C) 1998 Academic Press.