Tyrosin-protein kinase Lck (or lymphocyte-specific protein tyrosine kinase) is a 56 kDa protein that is found inside lymphocytes and encoded in the human by the LCK gene. The Lck is a member of Src kinase family (SKF) and is important for the activation of T-cell receptor (TCR) signaling in both naive T cells and effector T cells. The role of Lck is less prominent in the activation or in the maintenance of memory CD8 T cells in comparison to CD4 T cells. In addition, the constitutive activity of the mouse Lck homolog varies among memory T cell subsets. It seems that in mice, in the effector memory T cell (TEM) population, more than 50% of Lck is present in a constitutively active conformation, whereas less than 20% of Lck is present as active form in central memory T cells. These differences are due to differential regulation by SH2 domain–containing phosphatase-1 (Shp-1) and C-terminal Src kinase. Lck is responsible for the initiation of the TCR signaling cascade inside the cell by phosphorylating immunoreceptor tyrosine-based activation motifs (ITAM) within the TCR-associated chains. Lck can be found in different forms in immune cells: free in the cytosol or bound to the plasma membrane (PM) through myristoylation and palmitoylation. Due to the presence of the conserved CxxC motif (C20 and C23) in the zinc clasp structure, Lck is able to bind the cell surface coreceptors CD8 and\or CD4. Bound and free Lck have different properties: free Lck has more pronounced kinase activity in comparison to bound Lck, and moreover, the free form produces a higher level of T cell activation. The reasons for these differences are not well understood yet.
T cell signaling Lck is most commonly found in T cells. It associates with the cytoplasmic tails of the CD4 and CD8 co-receptors on T helper cells and cytotoxic T cells, respectively, to assist signaling from the T cell receptor (TCR) complex. T cells are able to respond to pathogen and cancer using T-cell receptor, nevertheless, they can also react to self-antigen causing the onset of autoimmune diseases. The T cells maturation occurs in the thymus and it is regulated by a threshold that defines the limit between the positive and the negative selection of thymocytes. in order to avoid the onset of autoimmune diseases, highly self-reactive T cells are removed during the negative selection, whereas, an amount of weak self-reactive T cells is required to promote an efficient immune response, therefore during the positive selection these cells are chosen for maturation. The threshold for positive and negative selection of developing T cells is regulated by the bound between the Lck and co-receptors. There are two main pools of T cells which mediate adaptive immune responses: CD4+ T cells (or helper T cells), and CD8+ T-cells (or cytotoxic T cells) which are MHCII-and MHCI restricted respectively. Despite their role in the immune system is different their activation is similar. Cytotoxic T cells are directly involved in the individuation and in the removal of infected cells, whereas helper T cells modulate other immune cells to supply the response. The initiation of immune response takes place when T cells encounter and recognize their cognate antigen. The antigen-presenting cells (APC) expose on their surface a fraction of the antigen that is recognized either from CD8+ T cells or CD4+ T cells. This binding leads to the activation of TCR signaling cascade in which the immunoreceptor tyrosine-based activation motifs (ITAM) located in the CD3-zeta chains (ζ-chains) of the TCR complex, are phosphorylated by Lck and less extended by Fyn. Both coreceptor-bound and free Lck can phosphorylate the CD3 chains upon TCR activation, evidences suggest that the free form of Lck can be recruited and trigger the TCR signal faster than the coreceptor-bound Lck Additionally, upon T cell activation, a fraction of kinase active Lck, translocate from outside of lipid rafts (LR) to inside lipid rafts where it interacts with and activates LR-resident Fyn, which is involved in further downstream signaling activation. Once ITAM complex is phosphorylated the CD3 chains can be bound by another cytoplasmic tyrosine kinase called ZAP-70. In the case of CD8+ T cells, once ZAP70 binds CD3, the coreceptor associated with Lck binds the MHC stabilizing the TCR-MHC-peptide interaction. The phosphorylated form of ZAP-70 recruits another molecule in the signaling cascade called LAT (Linker for activation of T cells), a transmembrane protein. LAT acts as a scaffold able to regulate the TCR proximal signals in a phosphorylation-dependent manner. The most important proteins recruited by phosphorylated LAT are Shc-Grb2-SOS, PI3K, and phospholipase C (PLC). The residue responsible for the recruitment of phospholipase C-γ1 (PLC-γ1) is Y132. This binding leads to the Tec family kinase ITK-mediated PLC-γ1 phosphorylation and activation that consequentially produce calcium (Ca2+) ions mobilization, and activation of important signaling cascades within the lymphocyte. These include the Ras-MEK-ERK pathway, which goes on to activate certain transcription factors such as NFAT, NF-κB, and AP-1. These transcription factors regulate the production of a plethora of gene products, most notable, cytokines such as Interleukin-2 that promote long-term proliferation and differentiation of the activated lymphocytes. In addition to the significance of Lck and Fyn in T cell receptor signaling, these two src kinases have also been shown to be important in TLR-mediated signaling in T cells. The function of Lck has been studied using several biochemical methods, including gene knockout (knock-out mice), Jurkat cells deficient in Lck (JCaM1.6), and siRNA-mediated RNA interference.
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