TAO (thousand-and-one amino acid) protein kinases mediate signaling from carbachol to p38 mitogen-activated protein kinase and ternary complex factors

The TAO (for thousand-and-one amino acids) protein kinases activate p38 mitogen-activated protein (MAP) kinase cascades in vitro and in cells by phosphorylating the MAP/ERK kinases (MEKs) 3 and 6. We found that TAO2 activity was increased by carbachol and that carbachol and the heterotrimeric G protein Gα_o could activate p38 in 293 cells. Using dominant interfering kinase mutants, we found that MEKs 3 and 6 and TAOs were required for p38 activation by carbachol or the constitutively active mutant Gα_oQ205L. To explore events downstream of TAOs, the effects of TAO2 on ternary complex factors (TCFs) were investigated. Transfection studies demonstrated that TAO2 stimulates phosphorylation of the TCF Elk1 on the major activating site, Ser³⁸³, and that TAO2 stimulates transactivation of Elk1 and the related TCF, Sap1. Reporter activity was reduced by the p38-selective inhibitor SB203580. Taken together, these studies suggest that TAO protein kinases relay signals from carbachol through heterotrimeric G proteins to the p38 MAP kinase, which then activates TCFs in the nucleus.