G alpha sub(i) is Not Required for Chemotaxis Mediated by G sub(i)-coupled Receptors

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