N-terminal Catalytic domain of Class III myosin-like Serine/Threonine Kinases
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. Class III myosins are motor proteins with an N-terminal kinase catalytic domain and a C-terminal actin-binding motor domain. Class III myosins are present in the photoreceptors of invertebrates and vertebrates and in the auditory hair cells of mammals. The kinase domain of myosin III can phosphorylate several cytoskeletal proteins, conventional myosin regulatory light chains, and can autophosphorylate the C-terminal motor domain. Myosin III may play an important role in maintaining the structural integrity of photoreceptor cell microvilli. It may also function as a cargo carrier during light-dependent translocation, in photoreceptor cells, of proteins such as transducin and arrestin. The Drosophila class III myosin, called NinaC (Neither inactivation nor afterpotential protein C), is critical in normal adaptation and termination of photoresponse. Vertebrates contain two isoforms of class III myosin, IIIA and IIIB. This subfamily also includes mammalian NIK-like embryo-specific kinase (NESK), Traf2- and Nck-interacting kinase (TNIK), and mitogen-activated protein kinase (MAPK) kinase kinase kinase 4/6. MAP4Ks are involved in some MAPK signaling pathways by activating a MAPK kinase kinase. MAPK signaling cascades are important in mediating cellular responses to extracellular signals. The class III myosin-like subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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