The motile major sperm protein (MSP) from Ascaris suum is a symmetric dimer in solution.

The major sperm protein (MSP) of Ascaris suum mediates amoeboid motility by forming an extensive intermeshed system of cytoskeletal filaments analogous to that formed by actin in many amoeboid cells. We have used a combination of biochemical and NMR methods to show that, in contrast to actin, MSP exist in solution as a symmetrical dimer. This result has important implications for the mechanism of both MSP filament assembly and the recognition of different MSP isoforms in vivo.