Catalytic domain of the Protein Tyrosine Kinases, Src-related kinase lacking C-terminal regulatory tyrosine and N-terminal myristylation sites (Srm) and Breast tumor kinase (Brk)
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Srm and Brk (also called protein tyrosine kinase 6) are members of the Src subfamily of proteins, which are cytoplasmic (or non-receptor) PTKs. Brk has been found to be overexpressed in a majority of breast tumors. Src kinases in general contain an N-terminal SH4 domain with a myristoylation site, followed by SH3 and SH2 domains, a tyr kinase domain, and a regulatory C-terminal region containing a conserved tyr; they are activated by autophosphorylation at the tyr kinase domain, but are negatively regulated by phosphorylation at the C-terminal tyr by Csk (C-terminal Src Kinase). Srm and Brk however, lack the N-terminal myristylation sites. Src proteins are involved in signaling pathways that regulate cytokine and growth factor responses, cytoskeleton dynamics, cell proliferation, survival, and differentiation. The Srm/Brk subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.