Catalytic kinase domain, second repeat, of the Giant Serine/Threonine Kinase Obscurin
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. Obscurin, approximately 800 kDa in size, is one of three giant proteins expressed in vetebrate striated muscle, together with titin and nebulin. It is a multidomain protein composed of tandem adhesion and signaling domains, including 49 immunoglobulin (Ig) and 2 fibronectin type III (FN3) domains at the N-terminus followed by a more complex region containing more Ig domains, a conserved SH3 domain near a RhoGEF and PH domains, non-modular regions, as well as IQ and phosphorylation motifs. The obscurin gene also encode two kinase domains, which are not expressed as part of the 800 kDa protein, but as a smaller, alternatively spliced product present mainly in the heart muscle, also called obscurin-MLCK. Obscurin is localized at the peripheries of Z-disks and M-lines, where it is able to communicate with the surrounding myoplasm. It interacts with diverse proteins including sAnk1, myosin, titin, and MyBP-C. It may act as a scaffold for the assembly of elements of the contractile apparatus. The obscurin subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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