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Mitogen-induced signal transduction is mediated by a cascade of protein phosphorylation and dephosphorylation. One of the immediate responses of mitogen stimulation is the activation of a family of protein kinases known as mitogen-activated protein kinase or extracellular signal-regulated kinase (ERK). MEK (MAP kinase or ERK kinase) is the immediate upstream activator kinase of ERK. Two cDNAs, MEK1 and MEK2, were cloned and sequenced. MEK1 and MEK2 encode 393 and 400 amino acid residues, respectively. The human MEK1 shares 99% amino acid sequence identity with the murine MEK1 and 80% with human MEK2. Both MEK1 and MEK2 were expressed in Escherichia coli and shown to be able to activate recombinant human ERK1 in vitro. The purified MEK2 protein stimulated threonine and tyrosine phosphorylation on ERK1 and concomitantly activated ERK1 kinase activity more than 100-fold. The recombinant MEK2 showed lower activity as an ERK activator as compared with MEK purified from tissue. However, the recombinant MEK2 can be activated by serum-stimulated cell extract in vitro. MEKs, in a manner similar to ERKs, are likely to consist of a family of related proteins playing critical roles in signal transduction.
Zheng et al. (Sat,) studied this question.