A post-docking role for active zone protein Rim

Rim1 was previously identified as a Rab3 effector localized to the presynap tic active zone in vertebrates. Here we demonstrate that C. elegans unc-10 mutants lacking Rim are viable, but exhibit behavioral and physiological de fects that are more severe than those of Rab3 mutants. Rim is localized to synaptic sites in C. elegans, but the ultrastructure of the presynaptic den sities is normal in Rim mutants. Moreover, normal levels of docked synaptic vesicles were observed in mutants, suggesting that Rim is not involved in the docking process. The level of fusion competent vesicles at release site s was reduced fivefold in Rim mutants, but calcium sensitivity of release e vents was unchanged. Furthermore, expression of a constitutively open form of syntaxin suppressed the physiological defects of Rim mutants, suggesting Rim normally acts to regulate conformational changes in syntaxin. These da ta suggest Rim acts after vesicle docking likely via regulating priming.