Editors' ChoiceCell Biology

G protein activation at the Golgi

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Science Signaling  28 Apr 2015:
Vol. 8, Issue 374, pp. ec109
DOI: 10.1126/scisignal.aab4170

Most heterotrimeric G protein activation occurs at the plasma membrane in response to ligand binding to G protein–coupled receptors (GPCRs) that function as guanine nucleotide exchange factors (GEFs) for the α subunit of the Gαβγ heterotrimer. However, some G proteins have been found associated with the Golgi, an intracellular network of membranes that receives proteins from the endoplasmic reticulum (ER) and acts as a processing and sorting hub for these proteins before they are trafficked to their destination. Proteins are transported to the Golgi from ER in vesicles coated with COPI, and fusion of these vesicles with the Golgi requires the small monomeric guanosine triphosphatase (GTPase) Arf1 and its GEFs and GTPase-activating proteins (GAPs). Lo et al. identified Gα-interacting vesicle-associated protein (GIV, also known as Girdin), which is not GPCR, as a GEF for Gαi at the Golgi. In COS7 cells, endogenous GIV and a tagged form of Gαi3 colocalized to the Golgi, and knockdown of GIV prevented the dissociation of tagged Gαi1 from Gβγ at the Golgi. Endogenous GIV cofractionated with COPI-positive membranes in COS7 cells, and tagged GIV coimmunoprecipitated with the COPI subunit β-COP from HeLa cell extracts. Golgi structure was altered, trafficking through the Golgi was impaired, and COPI vesicles accumulated in GIV-depleted cells or in cells expressing a GEF-defective GIV mutant, suggesting that the GEF activity of GIV was required for COPI vesicle uncoating. GIV-depleted cells also had increased amounts of GTP-bound (active) Arf1 and less ArfGAP3 associated with COPI vesicles. GIV bound to ArfGAPs 2 and 3 in pull-down assays, suggesting that GIV may stimulate the GTPase cycle of Arf1 by recruiting ArfGAP3 to COPI vesicles. Experiments using a peptide that binds Gβγ, which would both increase the amount of active Gα and sequester Gβγ, indicated that the GIV-mediated activation of the heterotrimeric G protein was important for GIV-mediated down-regulation of Arf1 activity, vesicle transport, and maintenance of Golgi architecture. An N-terminal region of GIV mediated binding to GTP-bound Arf1. These findings suggest that GTP-bound Arf1 recruits GIV to COPI vesicles and that GIV-mediated recruitment of ArfGAPs to COPI vesicles and activation of Gi at the Golgi surface inhibits Arf1 to stimulate vesicle uncoating and fusion with the Golgi membrane. Thus, GIV links heterotrimeric G proteins to the monomeric G protein Arf1 and provides evidence for Gαi activation at a site other than the plasma membrane.

I. -C. Lo, V. Gupta, K. K. Midde, V. Taupin, I. Lopez-Sanchez, I. Kufareva, R. Abagyan, P. A. Randazzo, M. G. Farquhar, P. Ghosh, Activation of Gαi at the Golgi by GIV/Girdin imposes finiteness in Arf1 signaling. Dev. Cell. 33, 189–203 (2015). [PubMed]

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