Mitogen-activated protein/ERK kinase kinases (MEKKs) phosphorylate and activate protein kinases which in turn phosphorylate and activate the p42/44 mitogen-activated protein kinase (MAPK), c-Jun/stress-activated protein kinases (JNKs), and p38/Hog1 kinase. We have isolated the cDNAs for two novel mammalian MEKKs (MEKK 2 and 3). MEKK 2 and 3 encode proteins of 69.7 and 71 kDa, respectively. The kinase domains encoded in the COOH-terminal moiety are 94% conserved; the NH2-terminal moieties are approximately 65% homologous, suggesting this region may encode sequences conferring differential regulation of the two kinases. Expression of MEKK 2 or 3 in HEK293 cells results in activation of p42/44MAPK and JNK but not of p38/Hog1 kinase. Immunoprecipitated MEKK 2 phosphorylated the MAP kinase kinases, MEK 1, and JNK kinase. Titration of MEKK 2 and 3 expression in transfection assays indicated that MEKK 2 preferentially activated JNK while MEKK 3 preferentially activated p42/44MAPK. These findings define a family of MEKK proteins capable of regulating sequential protein kinase pathways involving MAPK members.
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