Abstract
Mitogen-activated protein kinases (MAPKs) are activated through the kinase cascades of MAPK, MAPK kinase (MAPKK), and MAPKK kinase (MAPKKK). MAPKKK proteins relay upstream signals through the MAPK cascades to induce cellular responses. However, the molecular mechanisms by which given MAPKKKs are regulated remains largely unknown. Here, we found that a serine-threonine protein kinase (kinase X) physically interacted with the MAPKKK members MEKK1 and MEKK2 (MEKK1/2). The COOH-terminus, including the catalytic domain, but not the NH2-terminus of MEKK1/2 was necessary for the interaction with kinase X. Kinase X inhibited MEKK1/2 activation without preventing MEKK1/2 binding to its substrate, SEK1. Most importantly, kinase X suppressed the autophosphorylation of MEKK2. On the other hand, the knockdown of Kinase X by transfection with kinase X shRNA expression vector enhanced the stress-induced phosphorylation of the MAPKKK targets, MKK3/6. Taken together, our results indicate that kinase X negatively regulates the activation of MEKK1/2 by direct interaction with MEKK1/2's catalytic domain, suggesting a novel mechanism for MEKK1/2 regulation.
