Mammalian members related to Saccharomyces cerevisiae serine/threonine kinase STE20 can be divided into two subfamilies based on their structure and function. The PAK subfamily is characterized by an N-terminal p21-binding domain (also known as CRIB domain), a C-terminal kinase domain, and is regulated by the small GTP-binding proteins Rac1 and Cdc42Hs. The second group is represented by the GCK-like members, which contain an N-terminal catalytic domain and lack the p21-binding domain. Some of them have been demonstrated to induce c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) cascade, while others have been shown to be activated by a subset of stress conditions or apoptotic agents, although little is known about their specific function. Here, we have identified a novel human STE20- related serine/threonine kinase, belonging to the GCK-like subfamily. This kinase does not induce the JNK/SAPK pathway, but, instead, inhibits the basal activity of JNK/SAPK, and diminishes its activation in response to human epidermal growth factor (EGF). Therefore, we designated this molecule JIK for JNK/SAPK-inhibitory kinase. The inhibition of JNK/SAPK signaling pathway by JIK was found to occur between the EGF receptor and the small GTP-binding proteins Rac1 and Cdc42Hs. In contrast, JIK does not activate nor does it inhibit ERK2, ERK6, p38, or ERK5. Furthermore, JIK kinase activity is not modulated by any exogenous stimuli, but, interestingly, it is dramatically decreased upon EGF receptor activation. Thus, JIK might represent the first member of the STE20 kinase family whose activity can be negatively regulated by tyrosine kinase receptors, and whose downstream targets inhibit, rather than enhance, JNK/SAPK activation.
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