Gα16 activates Ras by forming a complex with tetratricopeptide repeat 1 (TPR1) and Son of Sevenless (SOS)

Many G protein-coupled receptors (GPCRs) are known to modulate cell growth and differentiation by stimulating the extracellular signal-regulated protein kinases (ERKs). In growth factor signaling, ERKs are typically stimulated through an elaborate network of modules consisting of adaptors, protein kinases, and the small GTPase Ras. The mechanism by which G protein signals tap into the ERK signaling pathway has thus far remain elusive. Members of the Gq family of G proteins, in particular Gα16, have been shown to associate with tetratricopeptide repeat 1 (TPR1), an adaptor protein which preferentially binds to Ras. Here, we examined if TPR1 is indeed the missing link between Gα16 signaling and Ras activation. Expression of Gα16QL, a constitutively active mutant of Gα16, in HEK 293 cells led to the formation of GTP-bound Ras and the subsequent phosphorylation of ERK. Likewise, stimulation of endogenou G16-coupled CCR1 chemokine receptors produced the same responses in human erythroleukemia cells. siRNA-mediated knockdown of TPR1 or expression of a dominant negative mutant of TPR1 effectively abolished the ability of Gα16QL to induce Ras activation in HEK 293 cells. In contrast, these manipulations had no inhibitory effect on Gα16QL-induced activation of phospholipase Cβ. Gα16QL-induced phosphorylations of downstream targets including ERK, signal transducer and activator of transcription 3, and IκB kinase were significantly suppressed upon expression of the dominant negative mutant of TPR1. Furthermore, SOS2, a Ras guanine nucleotide exchange factor, was found to form a complex with TPR1 and Gα16QL. Expression of SOS2 enhanced Gα16QL-induced Ras activation and its subsequent signaling. Collectively, our results suggest that Gα16 regulates multiple signaling pathways by activating Ras through its association with TPR1, but TPR1 is not required for Gα16 to stimulate phospholipase Cβ.