The Activating Dual Phosphorylation of MAPK by MEK Is Nonprocessive

Activation of mitogen-activated protein kinases (MAPKs), also known as extracellular-signal-regulated kinases (ERKs), by MAPK/extracellular protein kinase kinases (MEKs) requires phosphorylation at two sites. The first step in MAPK activation by MEK must be the formation of a MEK x MAPK enzyme-substrate complex, followed by phosphorylation producing monophosphorylated MAPK (pMAPK). Subsequently, one of two events may occur. (1) MEK catalyzes the second and fully activating phosphorylation of MAPK, producing ppMAPK (a processive mechanism). (2) The complex of MEK x pMAPK dissociates before the second phosphorylation occurs, full activation requiring a reassociation of pMAPK with MEK (a nonprocessive or distributive mechanism). Simulations indicate that these two mechanisms predict different kinetics of MAPK activation. Specifically, the nonprocessive mechanism predicts that there will be a paradoxical decrease in the rate of MAPK activation as the MAPK concentration is increased. The present study uses p42 MAPK, also known as ERK2, and MEK1 as representatives of their respective classes of enzymes. We find that increasing the ERK2 concentration decreases the rate of activation by a mechanism which does not involve inhibition of MEK1 function. The accumulation of the active, doubly phosphorylated ERK2 (ppERK2) was directly assessed using a phosphorylation-state-specific antibody. The rate of accumulation of ppERK2 is decreased by increasing the ERK2 concentration. Therefore, the mechanism of ERK2 activation by MEK1 in vitro is nonprocessive.