Plays a role in the synaptic transmission as bifunctional linker that interacts simultaneously with RIMS1, RIMS2, CACNA1D and CACNA1B. Interacts with CACNA1D and CACNA1B, and potentially with other Ca(2+) channel alpha-1 isoforms (By similarity). Interacts with RIMS1 and RIMS2. Synaptic plasma membrane. The SH3 domains mediate binding to a proline-rich motif in RIMS1, RIMS2, CACNA1D and CACNA1B. Belongs to the RIMBP family.