Multiple structural elements in voltage-dependent Ca2+ channels support their inhibition by G proteins

Molecular determinants of Ca2+ channel responsiveness to inhibition by receptor-coupled G proteins were investigated in Xenopus oocytes. The inhibitory response of alpha(1B) (N-type) channels was much larger than alpha(1A) (P/Q-type) channels, while alpha(1C) (L-type) channels were unresponsive. Differences in both degree and speed of inhibition were accounted for by variations in inhibitor off-rate. We tested proposals that inhibitory G protein and Ca2+ channel beta subunits compete specifically at the I-II loop. G protein-mediated inhibition remained unaltered in alpha(1B) subunits containing a point mutation in the I-II loop segment critical for Ca2+ channel beta subunit binding, and in chimeras where the I-II loop of alpha(1B) was replaced with counterparts from alpha(1A) or alpha(1C) Full interconversion between modulatory behaviors of alpha(1B) and alpha(1A) was achieved only by swapping both motif I and the C-terminus in combination. Thus, essential structural elements for G protein modulation reside in multiple Ca2+ channel domains.