A rab3 homolog in sea urchin functions in cell division.

Rabs are monomeric GTP binding proteins belonging to the ras superfamily that function throughout the secretory pathway. Members of the rab3 family function in the final steps of the secretory pathway, vesicle fusion with the plasma membrane. In contrast to mammalian systems with several rab3 isoforms (rab3A-D), a single family member homologue of rab3 is present in the rapidly dividing cleavage stage sea urchin embryo that localizes to numerous vesicles enriched at the cell cortex. We hypothesized that whereas the contents of these rab3-positive vesicles may contribute to the embryonic extracellular matrix, the membrane and its constituent proteins may be important for other aspects of cell division. We tested the function of rab3 in cell division by the microinjection of either antibodies or competing effector domain peptides to interfere with its function. We found that perturbing rab3 function results in cessation of cell division, whereas cells injected with either heat-inactivated antibodies or control scrambled peptides develop as normal. Moreover, neither endocytosis nor general membrane topology are affected by rab3 perturbation. Thus, we conclude that rab3-associated vesicles and/or their contents are critical for cell division.