Caenorhabditis elegans spermatozoa assemble membrane proteins onto the surface at the tips of pseudopodial projections.

The crawling movement of nematode sperm, like that of many other crawling metazoan cells, is accompanied by movement of membrane components from the leading edge of the cell rearward. We used colloidal gold conjugates of monoclonal antibodies (CGP-ABY) to membrane proteins on Caenorhabditis elegans sperm to examine this surface movement by electron microscopy. Antibody binding sites on fixed sperm are distributed uniformly over the cell surface. However, blocking these sites on live sperm with unlabelled antibody or removing them with protease and then pulse-labelling the cell with CGP-ABY revealed that new antigen is assembled onto the surface at the tips of the stubby projections that stud the pseudopod surface. These proteins then move rearward rapidly so that the pseudopod surface pool of antigen is replaced within 2 min. The same pattern of surface movement was observed when live cells were labelled with CGP-ABY and then washed with buffer before fixation. Bound CGP-ABY was cleared first from the tips of the projections and subsequently from the entire pseudopod surface. These gold particles accumulated at the base of the pseudopod without moving onto the cell body or being internalized. We did, however, detect a pool of antigen in the pseudopod cytoplasm that may be available for assembly onto the pseudopod surface. We propose that the localized assembly of new membrane and its subsequent rearward movement may play an important role in sperm locomotion.