The Ca²⁺-dependent binding of calmodulin to an N-terminal motif of the heterotrimeric G protein β subunit

Ca²⁺ ion concentration changes are critical events in signal transduction. The Ca²⁺-dependent interactions of calmodulin (CAM) with its target proteins play an essential role in a variety of cellular functions. In this study, we investigated the interactions of G protein βγ subunits with CaM. We found that CaM binds to known βγ subunits and these interactions are Ca²⁺-dependent. The CaM-binding domain in Gβγ subunits is identified as Gβ residues 40-63. Peptides derived from the Gβ protein not only produce a Ca²⁺-dependent gel mobility shifting of CaM but also inhibit the CaM- mediated activation of CaM kinase II. Specific amino acid residues critical for the binding of Gβγ to CaM were also identified. We then investigated the potential function of these interactions and showed that binding of CaM to Gβγ inhibits the pertussis toxin-catalyzed ADP-ribosylation of Gαo subunits, presumably by inhibiting heterotrimer formation. Furthermore, we demonstrated that interaction with CaM has little effect on the activation of phospholipase C-β2 by Gβγ subunits, supporting the notion that different domains of Gβγ are responsible for the interactions of different effectors. These findings shed light on the molecular basis for the interactions of Gβγ with Ca²⁺-CaM and point to the potential physiological significance of these interactions in cellular functions.