The complete pathway for catalytic activation of the mitogen-activated protein kinase, ERK2

The mitogen-activated protein (MAP) kinase ERK2 is an essential signal tran sduction molecule that mediates extracellular signaling by all polypeptide growth factors. Full activation of ERK2 requires phosphorylation at both a threonine residue (Thr(183)) conserved in most protein kinases as well as a tyrosine residue (Tyr(185)) unique to members of the mitogen-activated pro tein kinase family. We have characterized the kinetic role of phosphorylati on at each site with respect to the overall activation mechanism, providing a complete picture of the reaction steps involved. Phosphorylation at Tyr( 185) serves to configure the ATP binding site, while phosphorylation at bot h residues is required to stabilize binding of the protein substrate, myeli n basic protein. Similar control mechanisms are employed to stabilize ATP a nd myelin basic protein in the phosphoryl group transfer reaction, accounti ng for the enormous increase in turnover rate. The mechanism of ERK2 activa tion is kinetically similar to that of the cell cycle control protein, cdk2 /cyclinA. Phosphorylation of Tyr(185) in ERK2 and association of cyclinA wi th cdk2 both serve to stabilize ATP binding. Subsequent phosphorylation of both enzymes on threonine serves to stabilize binding of the phosphoaccepto r substrate.