부산대학교 도서관

Synaptic transmission is regulated by G protein-coupled receptors whose activation releases G protein β γ subunits that modulate presynaptic Ca2+ channels. The sequence motif QXXER has been proposed to be involved in the interaction between G protein β γ subunits and target proteins including adenylyl cyclase 2. This motif is present in the intracellular loop connecting domains I and II (LI-II) of Ca2+ channel α 1A subunits, which are modulated by G proteins, but not in α 1C subunits, which are not modulated. Peptides containing the QXXER motif from adenylate cyclase 2 or from α 1A block G protein modulation but a mutant peptide containing the sequence AXXAA does not, suggesting that the QXXER-containing peptide from α 1A can competitively inhibit Gβ γ modulation. Conversion of the R in the QQIER sequence of α 1A to E as in α 1C slows channel inactivation and shifts the voltage dependence of steady-state inactivation to more positive membrane potentials. Conversion of the final E in the QQLEE sequence of α 1C to R has opposite effects on voltage-dependent inactivation, although the changes are not as large as those for α 1A. Mutation of the QQIER sequence in α 1A to QQIEE enhanced G protein modulation, and mutation to QQLEE as in α 1C greatly reduced G protein modulation and increased the rate of reversal of G protein effects. These results indicate that the QXXER motif in LI-II is an important determinant of both voltage-dependent inactivation and G protein modulation, and that the amino acid in the third position of this motif has an unexpectedly large influence on modulation by Gβ γ . Overlap of this motif with the consensus sequence for binding of Ca2+ channel β subunits suggests that this region of LI-II is important for three different modulatory influences on Ca2+ channel activity.