MEK-1 substrate is a peptide substrate for MEK (MAP kinase kinase) and tyrosine protein kinases such as Lck p56; Km for Lck p56 tyrosine phosphorylation 170 µM. Sequence corresponds to amino acids 172-190 containing Thr 183 and Tyr 185 within the regulatory sequence common to ERK 1 and ERK 2 MAP kinases (plus 2 COOH terminal Arg residues); also reactive with additional species, including cow.