Csk

The Csk (C-terminal Src kinase) family is a small family of non-receptor tyrosine kinases, with CSK and MATK (also called CHK) in humans. Both carry a compact SH3-SH2-kinase architecture but, unlike Src-family enzymes, lack an N-terminal fatty-acylation signal and a C-terminal regulatory tyrosine. Their essential function is to phosphorylate the conserved C-terminal tyrosine of Src-family kinases, such as Src tyrosine 527, creating an intramolecular SH2-bound autoinhibited conformation. Through this negative feedback, Csk-family enzymes restrain Src signaling in platelets, immune cells and epithelia. CSK is ubiquitous and essential for development, whereas MATK is largely confined to hematopoietic and neural lineages and can additionally inhibit Src through direct SH2 binding.