Mitogen-activated protein kinase kinase kinase kinase 5
protein serine/threonine kinase activity
A novel human SPS1/STE20 homologue, KHS, activates Jun N-terminal kinase.
STE20-homologous proteins have been implicated in mammalian MAP kinase pathways as important transducers of signals from p21 family GTPases. We have cloned a novel STE20 family member, which we call KHS for kinase homologous to SPS1/STE20, that encodes a kinase of 95 kD which is expressed in a variety of tissues. Transiently expressed fusion protein GST-KHS exhibits phosphotransferase activity toward a panel of test substrates, including myelin basic protein (MBP), which is phosphorylated by all known STE20 homologues. KHS is most closely related to another human STE20, GC kinase (74% similar in the catalytic domain), which has recently been placed upstream of the stress-activated MAP kinases (SAPKs/JNKs). KHS also activates JNK in transient coexpression experiments, suggesting a role for KHS in the stress response of fibroblasts. Characterization and comparison of the regulation of these two kinases will be important in elucidating MAP kinase signalling cascades.