Rab Interacting Molecules 2 and 3 Directly Interact with the Pore-Forming Ca V 1.3 Ca 2+ Channel Subunit and Promote Its Membrane Expression.

Rab interacting molecules (RIMs) are multi-domain proteins that positively regulate the number of Ca2+ channels at the presynaptic active zone (AZ). Several molecular mechanisms have been demonstrated for RIM-binding to components of the presynaptic Ca2+ channel complex, the key signaling element at the AZ. Here, we report an interaction of the C2B domain of RIM2 alpha and RIM3 gamma with the C-terminus of the pore-forming alpha-subunit of Ca(V)1.3 channels (Ca(V)1.3 alpha 1), which mediate stimulus-secretion coupling at the ribbon synapses of cochlear inner hair cells (IHCs). Co-expressing full-length RIM2 alpha with a Ca2+ channel complex closely resembling that of IHCs (Ca(V)1.3 alpha 1-Ca-V beta 2a) in HEK293 cells doubled the Ca2+-current and shifted the voltage-dependence of Ca2+ channel activation by approximately +3 mV. Co-expression of the short RIM isoform RIM3 gamma increased the Ca(V)1.3 alpha 1-Ca-V beta 2a-mediated Ca2+-influx in HEK293 cells, but disruption of RIM3 gamma in mice left Ca2+-influx in IHCs and hearing intact. In conclusion, we propose that RIM2 alpha and RIM3 gamma directly interact with the C-terminus of the pore-forming subunit of Ca(V)1.3 Ca2+ channels and positively regulate their plasma membrane expression in HEK293 cells.