Mutations on the Switch III region and the alpha3 helix of Galpha(16 )differentially affect receptor coupling and regulation of downstream effectors

BACKGROUND: Gα(16 )can activate phospholipase Cβ (PLCβ) directly like Gα(q). It also couples to tetratricopeptide repeat 1 (TPR1) which is linked to Ras activation. It is unknown whether PLCβ and TPR1 interact with the same regions on Gα(16). Previous studies on Gα(q )have defined two minimal clusters of amino acids that are essential for the coupling to PLCβ. Cognate residues in Gα(16 )might also be essential for interacting with PLCβ, and possibly contribute to TPR1 interaction and other signaling events. RESULTS: Alanine mutations were introduced to the two amino acid clusters (246–248 and 259–260) in the switch III region and α3 helix of Gα(16). Regulations of PLCβ and STAT3 were partially weakened by each cluster mutant. A mutant harboring mutations at both clusters generally produced stronger suppressions. Activation of Jun N-terminal kinase (JNK) by Gα(16 )was completely abolished by mutating either clusters. Contrastingly, phosphorylations of extracellular signal-regulated kinase (ERK) and nuclear factor κB (NF-κB) were not significantly affected by these mutations. The interactions between the mutants and PLCβ2 and TPR1 were also reduced in co-immunoprecipitation assays. Coupling between G(16 )and different categories of receptors was impaired by the mutations, with the effect of switch III mutations being more pronounced than those in the α3 helix. Mutations of both clusters almost completely abolished the receptor coupling and prevent receptor-induced Gβγ release. CONCLUSION: The integrity of the switch III region and α3 helix of Gα(16 )is critical for the activation of PLCβ, STAT3, and JNK but not ERK or NF-κB. Binding of Gα(16 )to PLCβ2 or TPR1 was reduced by the mutations of either cluster. The same region could also differentially affect the effectiveness of receptor coupling to G(16). The studied region was shown to bear multiple functionally important roles of G(16).