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Publication Abstract from PubMed

The activation of Galpha subunits of heterotrimeric G proteins by G protein-coupled receptors (GPCRs) is a critical event underlying a variety of biological responses. Understanding how G proteins are activated will require structural and biochemical analyses of GPCRs complexed to their G protein partners, together with structure-function studies of Galpha mutants that shed light on the different steps in the activation pathway. Previously, we reported that the substitution of a glycine for a proline at position 56 within the linker region connecting the helical and GTP-binding domains of a Galpha chimera, designated alphaT*, yields a more readily exchangeable state for guanine nucleotides. Here we show that GDP-GTP exchange on alphaT*(G56P), in the presence of the light-activated GPCR, rhodopsin (R*), is less sensitive to the beta1gamma1 subunit complex than to wild-type alphaT*. We determined the X-ray crystal structure for the alphaT*(G56P) mutant and found that the G56P substitution leads to concerted changes that are transmitted to the conformationally sensitive switch regions, the alpha4-beta6 loop, and the beta6 strand. The alpha4-beta6 loop has been proposed to be a GPCR contact site that signals to the TCAT motif and weakens the binding of the guanine ring of GDP, whereas the switch regions are the contact sites for the beta1gamma1 complex. Collectively, these biochemical and structural data lead us to suggest that alphaT*(G56P) may be adopting a conformation that is normally induced within Galpha subunits by the combined actions of a GPCR and a Gbetagamma subunit complex during the G protein activation event.

A constitutively active galpha subunit provides insights into the mechanism of g protein activation.,Singh G, Ramachandran S, Cerione RA Biochemistry. 2012 Apr 17;51(15):3232-40. Epub 2012 Apr 5. PMID:22448927^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.