Could play a significant role in the signal transduction of hematopoietic cells. May regulate tyrosine kinase activity of SRC-family members in brain by specifically phosphorylating their C-terminal regulatory tyrosine residue which acts as a negative regulatory site. It may play an inhibitory role in the control of T-cell proliferation. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Interacts with KIT. In platelets, 90% of MATK localizes to the membrane fraction, and translocates to the cytoskeleton upon thrombin stimulation. Most abundant in brain, and to a lesser extent in the spleen, the thymus and the liver. Also found in the T-cell lineage. Minor isoform. Belongs to the protein kinase superfamily. Tyr protein kinase family. CSK subfamily.