Gα16 interacts with tetratricopeptide repeat 1 (TPR1) through its β3 region to activate Ras independently of phospholipase Cβ signaling

Background: G protein-coupled receptors constitute the largest family of cell surface receptors in the mammalian genome. As the core of the G protein signal transduction machinery, the G alpha subunits are required to interact with multiple partners. The GTP-bound active state of many G alpha subunits can bind a multitude of effectors and regulatory proteins. Yet it remains unclear if the different proteins utilize distinct or common structural motifs on the G alpha subunit for binding. Using G alpha(16) as a model, we asked if its recently discovered adaptor protein tetratricopeptide repeat 1 (TPR1) binds to the same region as its canonical effector, phospholipase C beta (PLC beta). Results: We have examined the specificity of G alpha 16/TPR1 association by testing a series of chimeras between G alpha(16) and G alpha(z). TPR1 co-immunoprecipitated with G alpha(16) and more tightly with its constitutively active G alpha(16)QL, but not G alpha(z). Progressive replacement of G alpha(16) sequence with the corresponding residues of G alpha(z) eventually identified a stretch of six amino acids in the beta 3 region of G alpha(16) which are responsible for TPR1 interaction and the subsequent Ras activation. Insertion of these six residues into G alpha(z) allowed productive TPR1-interaction. Since the beta 3 region only minimally contributes to interact with PLC beta, several chimeras exhibited differential abilities to stimulate PLC beta and Ras. The ability of the chimeras to activate downstream transcription factors such as signal transducer and activator of transcription 3 and nuclear factor kappa B appeared to be associated with PLC beta signaling. Conclusions: Our results suggest that G alpha(16) can signal through TPR1/Ras and PLC beta simultaneously and independently. The beta 3 region of G alpha(16) is essential for interaction with TPR1 and the subsequent activation of Ras, but has relatively minor influence on the PLC beta interaction. G alpha(16) may utilize different structural domains to bind TPR1 and PLC beta.