Rab Interacting Molecules 2 and 3 Directly Interact with the Pore-Forming Ca1.3 Ca Channel Subunit and Promote Its Membrane Expression.

Rab interacting molecules (RIMs) are multi-domain proteins that positively regulate the number of Ca channels at the presynaptic active zone (AZ). Several molecular mechanisms have been demonstrated for RIM-binding to components of the presynaptic Ca channel complex, the key signaling element at the AZ. Here, we report an interaction of the CB domain of RIM2α and RIM3γ with the C-terminus of the pore-forming α-subunit of Ca1.3 channels (Ca1.3α1), which mediate stimulus-secretion coupling at the ribbon synapses of cochlear inner hair cells (IHCs). Co-expressing full-length RIM2α with a Ca channel complex closely resembling that of IHCs (Ca1.3α1-Caß2a) in HEK293 cells doubled the Ca-current and shifted the voltage-dependence of Ca channel activation by approximately +3 mV. Co-expression of the short RIM isoform RIM3γ increased the Ca1.3α1-Caß2a-mediated Ca-influx in HEK293 cells, but disruption of RIM3γ in mice left Ca-influx in IHCs and hearing intact. In conclusion, we propose that RIM2α and RIM3γ directly interact with the C-terminus of the pore-forming subunit of Ca1.3 Ca channels and positively regulate their plasma membrane expression in HEK293 cells.