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A novel homologue of p38 MAP kinase, called SAPK4, has been cloned which shares 61% amino acid identity with p38 and is expressed predominantly in testes, pancreas and small intestine. We also cloned an alternative form of p38β, termed p38β2, which lacks the additional 8 amino acid insertion unique to p38β. p38, p38β, p38β2, ERK6/p38γ/SAPK3, and SAPK4 were characterized with respect to stimulus-dependent activation in transfected cells, substrate specificity, and sensitivity to inhibition by pyridinyl imidazoles. All homologues were stimulated, although to differing extents, by IL-1β, TNF, sorbitol, and UV. Only SAPK3 and SAPK4 were stimulated significantly by PMA. p38β showed the weakest activation overall. MBP, ATF-2, and both MAPKAP kinase-2 and kinase-3 were good substrates of p38 and p38βin vitro.In contrast, only MBP, ATF2, and MAPKAP kinase-3 proved to be significant substrates of …