EXPRESSION, PURIFICATION AND CHARACTERIZATION OF RECOMBINANT MITOGEN-ACTIVATED PROTEIN-KINASE KINASES

Mitogen-activated protein (MAP) kinase kinases (MKKs) are dual-specifi city protein kinases which activate p42(mapk) and p44(mapk) by phospho rylation of regulatory tyrosine and threonine residues. cDNAs for two isotypes of MKK, MKK1 and MKK2, have been isolated from several specie s. Here we describe construction of recombinant baculoviruses for high -level expression of histidine-tagged rat MKK1 and MKK2, and procedure s for production of nearly homogeneous MKK1 and MKK2 fusion proteins, in both inactive and active forms. Coinfection of Sf9 cells with eithe r MKK1 or MKK2 virus together with recombinant viruses for Raf-1, pp60 (src) (Y527F) and c-Ha-Ras resulted in activations of 250-fold and 150 -fold for MKK1 and MKK2 respectively. Specific activities towards kina se-defective p42(mapk) were of the order of several hundred nanomoles of phosphate transferred/min per mg of MKK protein. The Michaelis cons tants for both enzymes were approx. 1 mu M. Preparations of activated MKK were apparently free of Raf-1 as assessed by Western blotting. Raf -1 phosphorylated MKK1 on one major tryptic phosphopeptide, the phosph orylation of which increased with time. This phosphopeptide contained only phosphoserine and possessed neutral overall charge at pH 1.9 on t wo-dimensional peptide mapping. Phosphorylation of MKK1 by Raf-1 corre lated with activation and reached a plateau of similar to 2 mol/mol.