ACTIVATION OF THE NOVEL STRESS-ACTIVATED PROTEIN-KINASE SAPK4 BY CYTOKINES AND CELLULAR STRESSES IS MEDIATED BY SKK3 (MKK6) - COMPARISON OFITS SUBSTRATE-SPECIFICITY WITH THAT OF OTHER SAP KINASES

A cDNA was cloned that encodes human stress-activated protein kinase-4 (SAPK4), a novel MAP kinase family member whose amino acid sequence i s similar to 60% identical to that of the other three SAP kinases whic h contain a TGY motif in their activation domain. The mRNA encoding SA PK4 was found to be widely distributed in human tissues. When expresse d in KB cells, SAPK4 was activated in response to cellular stresses an d pro-inflammatory cytokines, in a manner similar to other SAPKs. SAPK 4 was activated in vitro by SKK3 (also called MKK6) or when co-transfe cted with SKK3 into COS cells. SKK3 was the only activator of SAPK4 th at was induced when KB cells were exposed to a cellular stress or stim ulated with interleukin-1. These findings indicate that SKK3 mediates the activation of SAPK4. The substrate specificity of SAPK4 in vitro w as similar to that of SAPK3. Both enzymes phosphorylated the transcrip tion factors ATF2, Elk-1 and SAP-1 at similar rates, but were far less effective than SAPK2a (also called RK/p38) or SAPK2b (also called p38 beta) in activating MAPKAP kinase-2 and MAPKAP kinase-3. Unlike SAPK1 (also called JNK), SAPK3 and SAPK4 did not phosphorylate the activati on domain of c-Jun. Unlike SAPK2a and SAPK2b, SAPK4 and SAPK3 were not inhibited by the drugs SB 203580 and SB 202190. Our results suggest t hat cellular functions previously attributed to SAPK1 and/or SAPK2 may be mediated by SAPK3 or SAPK4.