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 mu M, and the best substrate for p70(S6K) (KKRNRTLSVA) with a K-m of 1.5 mu M. 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.