Gα(i) is not required for chemotaxis mediated by G(i)-coupled receptors

Pertussis toxin inhibits chemotaxis of neutrophils by preventing chemoattractant receptors from activating trimeric G proteins in the G(i) subfamily. In HEK293 cells expressing recombinant receptors, directional migration toward appropriate agonist ligands requires release of free G protein βγ subunits and can be triggered by agonists for receptors coupled to G(i) but not by agonists for receptors coupled to two other G proteins, G(s) and G(q). Because activation of any G protein presumably releases free Gβγ, we tested the hypothesis that chemotaxis also requires activated α subunits (Gα(i)) of G(i) proteins. HEK293 cells were stably cotransfected with the G(i)-coupled receptor for interleukin-8, CXCR1, and with a chimeric Gα, Gα(qz5), which resembles Gα(i) in susceptibility to activation by G(i)-coupled receptors but cannot regulate the Gα(i) effector, adenylyl cyclase. These cells, unlike cells expressing CXCR1 alone, migrated toward interleukin-8 even after treatment with pertussis toxin, which prevents activation of endogenous Gα(i) but not that of Gα(qz5). We infer that chemotaxis does not require activation of Gα(i). Because chemotaxis is mediated by Gβγ subunits released when G(i)-coupled receptors activate Gα(qz5), but not when G(q)- or G(s)-coupled receptors activate their respective G proteins, we propose that G(i)-coupled receptors transmit a necessary chemotactic signal that is independent of Gα(i).