Rab3a binding and secretion-enhancing domains in Rim1 are separate and unique - Studies in adrenal chromaffin cells

Rim1 was identified in brain by its ability to bind Rab3a-GTP and has been postulated to be a Rab3a effector protein. Like Rabphilin3, it modulates secretion and contains a zinc finger and two C2 domains. We have investigated the structural basis for the ability of Rim1 to bind Rab3a-GTP and to stimulate exocytosis in chromaffin cells. Both full-length and N-terminal Rim1 enhance secretion 40-50% in both intact and permeabilized cells. The abilities of Rim1 to enhance secretion and to bind Rab3a-GTP reside on distinct and relatively small domains that act independently. A similar to 30-amino acid sequence immediately N-terminal of the zinc finger constitutes the minimal Rab3a-GTP binding domain. This short sequence is not found in Rabphilin3 and is entirely different from the zinc finger and flanking regions of Rabphilin3 that bind Rab3a-GTP, The zinc finger domain in Rim1 is unnecessary for Rab3a-GTP binding but, alone, enhances secretion. An analysis of the characteristics of the enhancement of secretion in permeabilized chromaffin cells indicates that N-terminal Rim1 does not alter the sensitivity of secretion to Ca2+ but, instead, increases the rate of ATP-dependent priming of secretion.