The main members of the mitogen-activated protein kinase (MAPK) family, extracellular signal–regulated kinases (ERKs) 1 and 2, c-Jun N-terminal kinase, and p38, are well characterized and regulate such cellular responses as proliferation and differentiation. In contrast, less is known about the regulation of the more recent additions to the MAPK family, such as ERK5, which is activated by various growth factor receptors, cytokine receptors, and Gq-coupled G protein–coupled receptors (GPCRs). In response to epidermal growth factor, activation of ERK5 requires the MAPK kinase MEK5 and the atypical protein kinase C isoform PKCζ; however, how ERK5 is activated by GPCRs is unclear. García-Hoz et al. found that stimulation of m1 muscarinic acetylcholine receptors led to the phosphorylation and activation of ERK5 in NIH 3T3 cells, which was independent of Src family kinases, EGF receptors, and phospholipase C–β (PLC-β), an effector of Gαq that leads to the activation of ERK1/2. Stimulation of another Gq-coupled GPCR led to the activation of ERK5 in wild-type, but not in PKCζ-deficient, mouse embryonic fibroblasts, and coimmunoprecipitation studies showed that Gαq physically associated with PKCζ. Immunoprecipitated complexes of tagged Gαq and PKCζ proteins contained MEK5. MEK5 and PKCζ contain PB1 domains through which they can interact; however, a PB1 domain–deficient mutant of MEK5 was found in immunoprecipitated complexes of Gαq and PKCζ. In addition to suggesting roles for MEK5 and PKCζ as effectors of Gαq, this study also suggests that Gαq acts as a scaffold protein that independently binds to MEK5 and PKCζ to mediate the activation of ERK5.
C. García-Hoz, G. Sánchez-Fernández, M. T. Díaz-Meco, J. Moscat, F. Mayor, C. Ribas, Gαq acts as an adaptor protein in protein kinase Cζ (PKCζ)-mediated ERK5 activation by G protein-coupled receptors (GPCR). J. Biol. Chem. 285, 13480–13489 (2010). [Abstract] [Full Text]