Boursin, a sea urchin bimC kinesin protein, plays a role in anaphase and cytokinesis

We have isolated and characterized Boursin, a kinesin-related protein of th e bimC family, from Paracentrotus lividus sea urchin eggs. Boursin is expre ssed at high levels in eggs and embryos during early cleavage stages. Bours in was found to be associated with different parts of the mitotic spindle f rom early prophase to telophase, Expression of a form of the protein predic ted to act as a dominant negative mutant caused severe defects in cell divi sion and resulted in the formation of embryos with polyploid and multiastra l blastomeres. Immunofluorescence analysis indicated that these defects did not arise from failure in either centrosome separation or bipolar spindle formation, Time-lapse observations showed rather that these perturbations i n cell division resulted from abnormal anaphase and failure to complete cyt okinesis. These phenotypes differ from the phenotype described following pe rturbation of the function of bimC family members in other organisms. Our s tudy has thus uncovered roles for a bimC kinesin in late stages of cell div ision.