Lck

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In immunology, the enzyme Lck (or leukocyte-specific protein tyrosine kinase) is a 56 kilodalton tyrosine kinase, associated with the cytoplasmic tails of the CD4 and CD8 co-receptors on T helper cells and cytotoxic T cells, respectively.

When the T cell receptor is engaged by the specific antigen presented by MHC, Lck phosphorylates the intracellular chains of the CD3 complex, allowing ZAP-70, a cytoplasmic protein tyrosine kinase, to bind. Lck then phosphorylates and activates ZAP-70, which in turn phosphorylates LAT (Linker of Activated T cells, a transmembrane protein serving as a docking site for a number of proteins, most importantly Shc-Grb2-SOS, PI3K and PLC).

The tyrosine phosphorylation cascade initiated by Lck culminates on the intracellular mobilization of a calcium 2+ ion and activation of the Ras-MEK-ERK pathway, thereby activating the transcription factors NFAT, NFκB and AP-1, which regulate the transcription of a plethora of gene products, most notably Interleukin-2, a cytokine which promotes long term proliferation and differentiation of activated lymphocytes.

Lck is inhibited by CBL, which is part of the ubiquitin mediated pathway.