Synergistic activation of SAPK1/JNK1 by two MAP kinase kinases in vitro

Mitogen-activated protein kinases (MAPKs) mediate many of the cellular effe cts of growth factors, cytokines and stress stimuli. Their activation requi res the phosphorylation of a threonine and a tyrosine residue located in a Thr-X-Tyr motif (where X is any amino acid) [1]. This phosphorylation is ca talysed by MAPK kinases (MKKs), which are all thought to be 'dual specifici ty' enzymes that phosphorylate both the threonine and the tyrosine residue of the Thr-X-Tyr motif [2], Here, we report that the MAPK family member kno wn as stress activated protein kinase-1c (SAPK1c, also known as JNK1) [3] i s activated synergistically in vitro by MKK4 ([4-6]; also called SKK1 and J NKK1) and MKK7 ([7-9]; also called SKK4 and JNKK2). We found that MKK4 had a preference for the tyrosine residue, and MKK7 for the threonine residue, within the Thr-X-Tyr motif. These observations suggest that the full activa tion of SAPK1c in vivo may sometimes require phosphorylation by two differe nt MKKs, providing the potential for integrating the effects of different e xtracellular signals. They also raise the possibility that other MAPK famil y members may be activated by two or more MKKs and that some MKKs may have gone undetected because they phosphorylate the tyrosine residue only, and t herefore do not induce any activation unless the threonine has first been p hosphorylated by another MKK.