Figure 1.

Recombinant constitutively active MEK1 protein phosphorylates ERK2 in vitro. Left, recombinant GST-MEK fusion proteins (wild-type = WT; kinase mutant = KM; and a constitutively active mutant in which Gln-56 was mutated to Pro = QP) were produced in E. coli and purified as described in MATERIALS AND METHODS. These proteins were then tested for activity in an in vitro kinase assay with [γ-³²P]ATP and a recombinant kinase-inactive ERK2 (ERK2^KR) as a substrate. Right, quantitation of ERK2^KR phosphorylation in the above assay by phosphorimage analysis.