Editors' ChoiceG Proteins

i-GDP Points the Way

Sci. Signal.  20 Aug 2013:
Vol. 6, Issue 289, pp. ec194
DOI: 10.1126/scisignal.2004643

When unstimulated, G protein–coupled receptors (GPCRs) associate with heterotrimeric G protein complexes composed of α, β, and γ subunits, in which the α subunit is bound to guanine diphosphate (GDP). Upon ligand binding, Gi-coupled GPCRs undergo a conformational change that stimulates the Gαi subunit to exchange the GDP for guanine triphosphate (GTP), thus triggering dissociation of the complex and liberation of Gαi-GTP and Gβγ, which can activate downstream signaling pathways. GTP hydrolysis by Gαi precedes reassociation of the subunits to regenerate the Gi holoenzyme. In migrating neutrophils, Gβγ released by chemoattractant-mediated activation of GPCRs stimulates signaling that culminates in the formation of F-actin in pseudopods at the leading edge to promote migration. Kamakura et al. report that the Gαi released by chemoattractant signaling also plays a role in neutrophil migration by targeting a polarity complex to the leading edge to maintain directionality. Using an affinity-tagged protein that binds specifically to Gαi-GDP, but not to Gαi-GTP or to Gαi-GDP in complex with Gβγ, the authors demonstrated that Gαi-GDP accumulated at the leading edge of chemoattractant-stimulated mouse neutrophils. Pertussis toxin inhibits the exchange of GDP for GTP by Gαi in response to GPCR activation and thus would be expected to trap the G protein in the heterotrimeric complex. Treating stimulated neutrophils with pertussis toxin prevented the accumulation of free Gαi-GDP, suggesting that GPCR-stimulated dissociation of Gαi-GTP from Gβγ followed by GTP hydrolysis produced the signaling-competent Gαi-GDP. Gαi-GDP binds to the polarity proteins LGN and AGS3, both of which interact with the polarity protein mInsc (mammalian homolog of Inscuteable). mInsc can simultaneously interact with LGN or AGS3 and the adaptor protein Par3, which is a component of the polarity complex that also contains Par6 and atypical protein kinase C (aPKC). In chemotaxing neutrophils, AGS3 and mInsc accumulated at the leading edge, and mInsc was required for pseudopod stabilization and maintenance of directionality but not for motility. These results suggest a model in which chemoattractant-mediated GPCR activation leads to accumulation of free Gαi-GDP, which recruits the polarity complex that helps neutrophils establish and maintain directionality while migrating.

S. Kamakura, M. Nomura, J. Hayase, Y. Iwakiri, A. Nishikimi, R. Takayanagi, Y. Fukui, H. Sumimoto, The cell polarity protein mInsc regulates neutrophil chemotaxis via a noncanonical G protein signaling pathway. Dev. Cell 26, 292–302 (2013). [PubMed]