Comparison of the specificities of p70 S6 kinase and MAPKAP kinase‐1 identifies a relatively specific substrate for p70 S6 kinase: the N‐terminal kinase domain of MAPKAP kinase‐1 is essential for peptide phosphorylation

xxR/KxRxxSxx sequences were phosphorylated with high efficiency by both p70 S6 kinase (p70S6K) and MAPKAP kinase-1. The best substrate for MAPKAP kinase-1 (KKKNRTLSVA) was phosphorylated with a Km of 0.17 microM, and the best substrate for p70S6K (KKRNRTLSVA) with a Km of 1.5 microM. The requirement of both enzymes for Arg/Lys at position n-5 could be partially replaced by inserting basic residues at other positions, especially by an Arg at n-2 or n-4. MAPKAP kinase-1 (but not p70S6K) tolerated lack of any residue at n-5 if Arg was present at n-2 and n-3. p70S6K (but not p90S6K) tolerated Thr at position n and absence of any residue at n + 2. The peptide KKRNRTLTV, which combined these features, was relatively selective for p70S6K having a 50-fold higher Vmax/Km than MAPKAP kinase-1. Inactivation of the N-terminal kinase domain of MAPKAP kinase-1, which is 60% identical to p70S6K, abolished activity towards all peptides tested, but the enzyme retained 30-40% of its activity if the C-terminal kinase domain was inactivated.