A rab3 homolog in sea urchin functions in cell division

Rabs are monomeric GTP binding proteins belonging to the ras superfamily th at function throughout the secretory pathway. Members of the rab3 family fu nction in the final steps of the secretory pathway, vesicle fusion with the plasma membrane, In contrast to mammalian systems with several rab3 isofor ms (rab3A-D), a single family member homologue of rab3 is present in the ra pidly dividing cleavage stage sea urchin embryo that localizes to numerous vesicles enriched at the cell cortex. We hypothesized that whereas the cont ents of these rab3-positive vesicles may contribute to the embryonic extrac ellular 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 ra b3 function results in cessation of cell division, whereas cells injected w ith either heat-inactivated antibodies or control scrambled peptides develo p as normal. Moreover, neither endocytosis nor general membrane topology ar e affected by rab3 perturbation, Thus, we conclude that rab3-associated ves icles and/or their contents are critical for cell division.