Dopamine Receptor-interacting Protein 78 Acts as a Molecular Chaperone for Gγ Subunits before Assembly with Gβ

Abstract Heterotrimeric G proteins play a central role in intracellular communication mediated by extracellular signals, and both Gα and Gβγ 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 Gβγ expression and subsequent signaling by chaperoning nascent Gβ and facilitating heterodimer formation with Gγ subunits (Lukov, G. L., Hu, T., McLaughlin, J. N., Hamm, H. E., and Willardson, B. M. (2005) EMBO J. 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 Gγ subunit but not Gβ or Gα subunits. Furthermore, we demonstrate that DRiP78 and the Gβ subunit can compete for the Gγ subunit. DRiP78 also protects Gγ from degradation until a stable partner such as Gβ is provided. Furthermore, DRiP78 interaction may represent a mechanism for assembly of specific Gβγ heterodimers, as selectivity was observed among Gγ isoforms for interaction with DRiP78 depending on the presence of particular Gβ subunits. Interestingly, we could detect an interaction between DRiP78 and PhLP, suggesting a role of DRiP78 in the assembly of Gβγ by linking Gγ to PhLP·Gβ complexes. Our results, therefore, suggest a role of DRiP78 as a chaperone in the assembly of Gβγ subunits of the G protein.