Construction of Gα₁₆̳/s̳ chimeras that link Gs̳-coupled receptors to the stimulation of PLC{221}

Heterotrimeric G proteins transduce extracellular signals detected by receptors into intracellular responses which underlie physiological responses of tissues and cells. Molecular, genetic and biochemical studies have identified distinct regions at the extreme C termini of the G protein α subunits that are of fundamental importance in determining the fidelity of receptor-G protein interactions. Of all the known Gα subunits, the promiscuous nature of Gαl6 permits it to interact with a wide range of G protein-coupled receptors. However several Gs-linked receptors (e.g. luteinizing hormone receptor) manifest subdued activation of phospholipase C (PLC) via Gα16. To enhance receptor coupling efficiency and delineate residues at the C-terminus of the Gαl6 subunit which play pivotal roles in determining selectivity of receptor recognition, a series of chimeric proteins were constructed in which specific regions of Gαl6 were replaced with cognate regions of Gαs. The altered regions include the α5 helix, β6 strand and the α4/β6 loop. Upon activation by several Gs-coupled receptors, agonist-induced stimulation of PLC was more efficiently mediated by the chimera 16s25 than Gα16, wherein 25 residues of the α5 helix of Gαl6 were replaced by that of Gαs. Higher basal activity without ligand stimulation was also observed. Another chimera, 16s45, also demonstrated higher maximal and lower EC50 values than Gαl6 on activation by the luteinizing hormone receptor. None of the receptors examined were able to stimulate PLC via chimera 16s81. These results can form the basis for further characterization of the α5 helix residues with the aim of mapping functionally relevant receptor-G protein contact sites. Whilst contributing to a better understanding of the molecular basis of receptor-G protein interactions, Gα16/s chimeras with expanded potential to interact with Gs-coupled receptors may be used to link orphan receptors to the stimulation of PLC