Direct binding of G-protein βλ complex to voltage-dependent calcium channels

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Abstract
Voltage-dependent Ca2+ channels play a central role in controlling neurotransmitter release at the synapse1,2. They can be inhibited by certain G-protein-coupled receptors, acting by a pathway intrinsic to the membrane3–6. Here we show that this inhibition results from a direct interaction between the G-protein βλ complex and the pore-forming α1 subunits of several types of these channels7. The interaction is mediated by the cytoplasmic linker connecting the first and second transmembrane repeats. Within this linker, binding occurs both in the α1 interaction domain (AID)8, which also mediates the interaction between the α1 and β subunits of the channel, and in a second downstream sequence. Further analysis of the binding site showed that several amino-terminal residues in the AID are critical for Gβλ binding, defining a site distinct from the carboxy-terminal residues shown to be essential for binding the β-subunit of the Ca2+ channel9. Mutation of an arginine residue within the N-terminal motif abolished βλ binding and rendered the channel refractory to G-protein modulation when expressed in Xenopus oocytes, showing that the interaction is indeed responsible for G-protein-dependent modulation of Ca2+ channel activity.