Go-protein α-Subunits Activate Mitogen-activated Protein Kinase via a Novel Protein Kinase C-dependent Mechanism

Abstract

Mitogen-activated protein kinase (MAPK) is activated in response to both receptor tyrosine kinases and G-protein-coupled receptors. Recently, Gi-coupled receptors, such as the α2A adrenergic receptor, were shown to mediate Ras-dependent MAPK activation via a pathway requiring G-protein βγ subunits (Gβγ) and many of the same intermediates involved in receptor tyrosine kinase signaling. In contrast, Gq-coupled receptors, such as the M1 muscarinic acetylcholine receptor (M1AChR), activate MAPK via a pathway that is Ras-independent but requires the activity of protein kinase C (PKC). Here we show that, in Chinese hamster ovary cells, the M1AChR and platelet-activating factor receptor (PAFR) mediate MAPK activation via the α-subunit of the Go protein. Go-mediated MAPK activation was sensitive to treatment with pertussis toxin but insensitive to inhibition by a Gβγ-sequestering peptide (βARK1ct). M1AChR and PAFR catalyzed Goα-subunit GTP exchange, and MAPK activation could be partially rescued by a pertussis toxin-insensitive mutant of Goα but not by similar mutants of Gi. Go-mediated MAPK activation was insensitive to inhibition by a dominant negative mutant of Ras (N17Ras) but was completely blocked by cellular depletion of PKC. Thus, M1AChR and PAFR, which have previously been shown to couple to Gq, are also coupled to Go to activate a novel PKC-dependent mitogenic signaling pathway. Mitogen-activated protein kinase (MAPK) is activated in response to both receptor tyrosine kinases and G-protein-coupled receptors. Recently, Gi-coupled receptors, such as the α2A adrenergic receptor, were shown to mediate Ras-dependent MAPK activation via a pathway requiring G-protein βγ subunits (Gβγ) and many of the same intermediates involved in receptor tyrosine kinase signaling. In contrast, Gq-coupled receptors, such as the M1 muscarinic acetylcholine receptor (M1AChR), activate MAPK via a pathway that is Ras-independent but requires the activity of protein kinase C (PKC). Here we show that, in Chinese hamster ovary cells, the M1AChR and platelet-activating factor receptor (PAFR) mediate MAPK activation via the α-subunit of the Go protein. Go-mediated MAPK activation was sensitive to treatment with pertussis toxin but insensitive to inhibition by a Gβγ-sequestering peptide (βARK1ct). M1AChR and PAFR catalyzed Goα-subunit GTP exchange, and MAPK activation could be partially rescued by a pertussis toxin-insensitive mutant of Goα but not by similar mutants of Gi. Go-mediated MAPK activation was insensitive to inhibition by a dominant negative mutant of Ras (N17Ras) but was completely blocked by cellular depletion of PKC. Thus, M1AChR and PAFR, which have previously been shown to couple to Gq, are also coupled to Go to activate a novel PKC-dependent mitogenic signaling pathway.