Concentration-dependent positive and negative regulation of a MAP kinase by a MAP kinase kinase

There are at least three distinct MAP kinase signaling modules in mammalian cells, distinguished by the family of kinases (Erk, SAPK/JNK, or p38) that is ultimately activated. Many input signals activate multiple MAP kinase c ascades, and the mechanisms that control the specificity of signal output a re not well understood. We show that SEK1/MKK4, a MAP kinase kinase propose d to activate SAPK/JNK, is a very potent inhibitor of p54 SAPK beta/JNK3 bo th in vitro and irt vivo if present at equimolar or higher ratios. In contr ast SEK can activate SAPK when present in substoichiometric amounts, but th is activation is slow, consistent with the rate-limiting step in activation being the dissociation of an inactive SEK:SAPK complex. The N-terminal uni que region of SEK is both necessary and partially sufficient for inhibition of SAPK, and is also necessary for activation of SAPK by SEK in vitro. We have also used the p38 MAP kinase and its activator MKK6 to examine the reg ulatory relationships among different kinases involved in stress responses. We show using purified kinases that inhibitory activity is specific for th e combination of SEK and SAPK: SEK can activate but not inhibit p38, and MK K6 can activate but not inhibit SAPK beta and p38. These results reveal a p otential mechanism for regulating stress-activated kinases, adding to a gro wing body of evidence suggesting that MAP kinases are controlled by relativ ely stable interactions with their activators.