Signaling from G protein-coupled receptors to ERK5/big MAPK 1 involves G alpha(q) and G alpha(12/13) families of heterotrimeric G proteins - Evidencefor the existence of a novel Ras and Rho-independent pathway

The regulation of gene expression by cell surface receptors often involves the stimulation of signaling pathways including one or more members of the MAPK superfamily of serine-threonine kinases. Upon their activation in the cytosol, MAPKs can translocate to the nucleus and affect the activity of a variety of transcription factors. Recently, it has been observed that a nov el member of the MAPK superfamily, ERK5, can be potently activated by trans forming G protein-coupled receptors (GPCRs) and that ERK5 participates in t he regulation of c-jun expression through the activation of MEF2 transcript ion factors. How cell surface receptors, including GPCRs, stimulate ERK5 is still poorly understood. In this study, we have used transiently transfect ed COS-7 cells to begin delineating the biochemical route linking GPCRs to ERK5. We show that receptors that can couple to the Gq and G(12/13) familie s of heterotrimeric G proteins, mi and thrombin receptors, respectively, bu t not those coupled to G(i), such as m2 receptors, are able to regulate the activity of ERK5. To investigate which heterotrimeric G proteins signal to ERK5, we used a chimeric system by which G alpha(q)- and G alpha(13)-media ted signaling pathways can be conditionally activated upon ligand stimulati on. Using this system, as well as the expression of activated forms of G pr otein subunits, we show that the G alpha(q) and G alpha(12/13) families of heterotrimeric G proteins, but not the G alpha(i), G alpha(s), and beta gam ma subunits, are able to regulate ERK5. Furthermore, we provide evidence th at the stimulation of ERK5 by GPCRs involves a novel signaling pathway, whi ch is distinct from those regulated by Ras and Rho GTPases.