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 (p70^S6K) and MAPKAP kinase-1. The best substrate for MAPKAP kinase-1 (KKKNRTLSVA) was phosphorylated with a K_m of 0.17 μM, and the best substrate for p70^S6K (KKRNRTLSVA) with a K_m of 1.5 μM. 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 p70^S6K) tolerated lack of any residue at n - 5 if Arg was present at n - 2 and n - 3. p70^S6K (but not p90^S6K) tolerated Thr at position n and absence of any residue at n + 2. The peptide KKRNRTLTV, which combined these features, was relatively selective for p70^S6K having a 50-fold higher V_max/K_m than MAPKAP kinase-1. Inactivation of the N-terminal kinase domain of MAPKAP kinase-1, which is 60% identical to p70^S6K, abolished activity towards all peptides tested, but the enzyme retained 30-40% of its activity if the C-terminal kinase domain was inactivated.