Scanning mutagenesis reveals a role for serine 189 of the heterotrimeric G-protein beta 1 subunit in the inhibition of N-type calcium channels

Direct interactions between the presynaptic N-type calcium channel and the beta subunit of the heterotrimeric G-protein complex cause voltage-dependent inhibition of N-type channel activity, crucially influencing neurotransmitter release and contributing to analgesia caused by opioid drugs. Previous work using chimeras of the G-protein beta subtypes G beta(1) and G beta(5) identified two 20-amino acid stretches of structurally contiguous residues on the G beta(1) subunit as critical for inhibition of the N-type channel. To identify key modulation determinants within these two structural regions, we performed scanning mutagenesis in which individual residues of the G beta(1) subunit were replaced by corresponding G beta(5) residues. Our results show that G beta(1) residue Ser(189) is critical for N-type calcium channel modulation, whereas none of the other G beta(1) mutations caused statistically significant effects on the ability of G beta(1) to inhibit N-type channels. Structural modeling shows residue 189 is surface exposed, consistent with the idea that it may form a direct contact with the N-type calcium channel alpha(1) subunit during binding interactions.