Rab3a interacting molecule (RIM) and the tethering of pre-synaptic transmitter release site-associated CaV2.2 calcium channels.

Biochemical and physiological evidence suggest that pre-synaptic calcium channels are attached to the transmitter release site within the active zone by a molecular tether. A recent study has proposed that 'Rab3a Interacting Molecule' (RIM) serves as the tether for CaV2.1 channels in mouse brain, based in part on biochemical co-immunoprecipitation (co-IP) using a monoclonal antibody, mRIM. We previously argued against this idea for CaV2.2 calcium channel at chick synapses based on experiments using a different anti-RIM antibody, pRIM1,2: while staining for the two proteins co-localized and co-varied at the transmitter release face, consistent with an association, they failed to co-IP from a synaptosome membrane lysate. RIM is, however, a family of proteins and we tested the possibility that the mRIM antibody used in the more recent study identifies a particular channel-tethering variant. We find that co-immunostaining with mRIM and anti-CaV2.2 antibody neither co-localized nor co-varied at the transmitter release face and the two proteins did not co-IP, arguing against a common protein complex and a key CaV2.2 scaffolding role for RIM at the active zone. The differing results might be reconciled, however, in a model where a RIM family member contributes to a protein bridge that anchors the pre-fusion secretory vesicle to the calcium channel protein complex.