Calcium entry through voltage?gated channels (VGCCs) initiates a variety of cellular processes. VGCCs are multi-protein complexes containing a pore-forming subunit (CaVa1) and a variable number of auxiliary subunits. The auxiliary b-subunit (CaVb) binds to the AID site, shared by all CaVa1, and strongly influences channel function. CaVb encompasses two protein- protein interaction domains: a Src homology 3 (CaVb-SH3) and a guanylate kinase (CaVb-GK) domain. While binding to AID occurs exclusively through CaVb-GK the functional significance of the SH-3 domain remains elusive. Here, we show that injection of isolated CaVb-SH3 domain into Xenopus oocytes expressing CaVa1 reduces the number of channels in the plasma membrane. Full length CaVb also downregulates VGCCs but only when lacking the AID site. Furthermore, CaVb-SH3 and the full length CaVb were found to interact in vitro with dynamin, a GTPase involved in receptor-mediated endocytosis and to internalize the distantly related Shaker potassium channel. Our study describes a novel interaction between CaVb and dynamin that is governed by the association state to CaVa1 and mediates membrane protein internalization.