Dopamine receptor-interacting protein 78 acts as a molecular chaperone for Gγ subunits before assembly with Gβ

Heterotrimeric G proteins play a central role in intracellular communication mediated by extracellular signals, and both G alpha and G beta gamma subunits regulate effectors downstream of activated receptors. The particular constituents of the G protein heterotrimer affect both specificity and efficiency of signal transduction. However, little is known about mechanistic aspects of G protein assembly in the cell that would certainly contribute to formation of heterotrimers of specific composition. It was recently shown that phosducin-like protein (PhLP) modulated both G beta gamma expression and subsequent signaling by chaperoning nascent G beta and facilitating heterodimer formation with G gamma subunits Lukov, G. L., Hu, T., McLaughlin, J. N., Hamm, H. E., and Willardson, B. M. ( 2005) EMBOJ. 24, 1965 - 1975; Humrich, J., Bermel, C., Bunemann, M., Harmark, L., Frost, R., Quitterer, U., and Lohse, M. J. ( 2005) J. Biol. Chem. 280, 20042 - 20050). Here we demonstrate using a variety of techniques that DRiP78, an endoplasmic reticulum resident protein known to regulate the trafficking of several seven transmembrane receptors, interacts specifically with the G gamma subunit but not G beta or G alpha subunits. Furthermore, we demonstrate that DRiP78 and the G beta subunit can compete for the G gamma subunit. DRiP78 also protects G gamma from degradation until a stable partner such as G beta is provided. Furthermore, DRiP78 interaction may represent a mechanism for assembly of specific G beta gamma heterodimers, as selectivity was observed among G gamma isoforms for interaction with DRiP78 depending on the presence of particular G beta subunits. Interestingly, we could detect an interaction between DRiP78 and PhLP, suggesting a role of DRiP78 in the assembly of G beta gamma by linking G gamma to PhLP center dot G beta complexes. Our results, therefore, suggest a role of DRiP78 as a chaperone in the assembly of G beta gamma subunits of the G protein.