Editors' ChoiceCell Biology

Gβ proteins as components of E3 ubiquitin ligases

Sci. Signal.  16 Jun 2015:
Vol. 8, Issue 381, pp. ec157
DOI: 10.1126/scisignal.aac7832

G protein–coupled receptors (GPCRs), such as the β-adrenergic receptors in the heart, relay signals through heterotrimeric G proteins composed of α, β, and γ subunits. Upon GPCR activation, the Gα and Gβγ subunits independently trigger downstream signaling events. Some Gβ subunits recruit GPCR kinases (GRKs) to the plasma membrane, where they phosphorylate the GPCR, which can lead to receptor desensitization, receptor degradation, or interaction with β-arrestins. Gβ proteins contain WD40 repeats, which bind to DDB1 and are a member of the family of proteins called DCAFs (DDB1 and CUL4–associated factors). The DDB1-CUL4A complex is a component of cullin RING E3 ubiquitin ligases (CRLs). Zha et al. found that the five Gβ proteins in HEK293 cells bound to CUL4A. Gβ2 bound to the DDB1-CUL4A complex independently of Gγ. Immunoprecipitation and Western blot analysis showed that CRL E3 ligase associated with Gβ2 (CRLGβ2) ubiquitylated GRK2 in HEK293 wild-type cells, whereas GRK2 bound to R214A mutant of Gβ2, expressed in HEK293 cells, which cannot bind CUL4A, did not get ubiquitylated. Exposing HEK293 cells to the β-adrenergic agonist, isoproterenol, reduced the interaction of Gβ2 with DDB1-CUL4A and increased GRK2 abundance. Mutational analysis of PKA (protein kinase A) phosphorylation sites on DDB1 indicated that isoproterenol-induced DDB1 phosphorylation on Ser645 disrupted the interaction of DDB1 with Gβ2. CUL4A−/− mice had hypertrophic hearts with increased abundance of GRK2; reducing the GRK2 dosage (CUL4A−/−; Grk+/−), GRK2 abundance was similar to that of wild-type mice and the heart weight to body weight ratio (Hw/Bw) was significantly less than the Hw/Bw of CUL4A−/−. Thus, this study showed that Gβ functions independently from Gγ as an adaptor component of the CRLGβ2 E3 ligase that may regulate GPCR signaling by targeting GRK2 for degradation.

Z. Zha, X. Han, M. D. Smith, Y. Liu, P. M. Giuere, D. Kopanja, P. Raychaudhuri, D. P. Siderovski, K. -L. Guan, Q. -Y. Lei, and Y. Xiong, A non-canonical function of Gβ as a subunit of E3 ligase in targeting GRK2 ubiquitylation. Mol. Cell. 58, 794–803 (2015). [PubMed]