In vitro filament formation by the major sperm protein (MSP) of Caenorhabditis elegans

Abstract

Nematode sperm are asymmetric cells which exhibit an amoeboid motility similar to that seen in other actin-rich eukaryotic cells. However, the pseudopod of these cells is not packed with actin, but with 2-5 nm fine filaments of major sperm protein (MSP), a family of small, basic sperm-specific polypeptides which constitute ∼15% of the total cellular protein of the sperm. In Caenorhabditis elegans, the MSPs are encoded by a large multigene family of ∼60 members which has made genetic analysis of MSP and its role in cell motility impossible. In this work, C. elegans pMSP 56 has been synthesized at high levels using an E. coli expression vector. Two methods of pMSP 56 purification have been developed, one resulting in 60-70% pure MSP and the second resulting in 99% pure MSP. Both of these purified proteins are also capable of assembling into 2.5 ± 0.4 nm diameter filaments in the presence of 30% ethanol. These filaments seem to have a definite substructure, although detailed analysis of the substructure has not been carried out. The ability to produce large amounts of pure MSP and its ability to assemble into filaments represents a first step in developing an in vitro assay system for the MSP cytoskeleton. (Abstract shortened by UMI.)