Protein-tyrosine kinase-6 (PTK6, also known as Brk) is a non-receptor tyrosine kinase that contains SH3, SH2, and catalytic (Kinase) domains. We have identified an intramolecular interaction between the linker (Linker) region connecting the SH2 and Kinase domains and the Kinase domain. Residue Trp-184 within the Linker region is essential for the Linker-Kinase interaction but not for the Linker-SH3 interaction. A recombinant PTK6 Kinase domain connected to the Linker region had catalytic

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... y in terms of autophosphorylation, phosphorylation of a PTK6 substrate, BKS, and phosphorylation of an oligopeptide substrate, whereas the Kinase domain itself, or one connected to a Linker region containing a W184A substitution, did not. The introduction of the W184A mutation into PTK6 also abrogated autophosphorylation and phosphorylation of another PTK6 substrate, Sam68, as well as phosphorylation of intracellular proteins. It also abolished the ability of PTK6 to promote proliferation and prevent apoptosis of HEK 293 cells, as well as to permit anchorage-independent colony formation. Therefore, unlike Src family members, in which the Linker-Kinase interaction inhibits catalytic activity, in PTK6 this interaction has an essential positive role. Protein-tyrosine kinase (PTK)-6, 1 also known as Brk, is a non-receptor PTK. A partial cDNA of PTK6 was first identified during an extensive survey of PTK mRNAs expressed in human melanocytes (1). A full-length human cDNA was cloned from breast carcinoma cells (2), and a cDNA for its mouse homolog, Sik, which has 80% amino acid identity to PTK6, was cloned from mouse intestinal crypt cells (3).