The Ca2+-dependent Binding of Calmodulin to an N-terminal Motif of the Heterotrimeric G Protein beta Subunit

Abstract

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 beta gamma subunits with CaM. We found that CaM binds to known beta gamma subunits and these interactions are Ca2+-dependent. The CaM-binding domain in Gbeta gamma subunits is identified as Gbeta residues 40-63. Peptides derived from the Gbeta 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 Gbeta gamma to CaM were also identified. We then investigated the potential function of these interactions and showed that binding of CaM to Gbeta gamma inhibits the pertussis toxin-catalyzed ADP-ribosylation of Galpha o subunits, presumably by inhibiting heterotrimer formation. Furthermore, we demonstrated that interaction with CaM has little effect on the activation of phospholipase C-beta 2 by Gbeta gamma subunits, supporting the notion that different domains of Gbeta gamma are responsible for the interactions of different effectors. These findings shed light on the molecular basis for the interactions of Gbeta gamma with Ca2+-CaM and point to the potential physiological significance of these interactions in cellular functions.