The Lck bound to the cytoplasmic tails of CD4 or CD8 is brought near its substrate ITAMs in the T-cell receptor when the co-receptor binds the recep-tor:peptide:MHC complex (see Fig. 7.11). Lck’s activity is also regulated allosterically by phosphorylation of a tyrosine in its carboxy terminus by the C-terminal Src kinase (Csk). The resulting phosphotyrosine interacts with Lck’s SH2 domain and helps maintain Lck in a closed conformation, resulting in a catalytically inactive state (Fig. 7.12). The absence of Csk during T-cell development causes T cells to mature autonomously in the thymus without needing to bind peptide:MHC, presumably as a result of abnormal activation of TCR signaling by hyperactive Lck in Csk-deficient thymocytes. This suggests that Csk normally acts to reduce Lck activity and to attenuate TCR signaling. Dephosphorylation of the tyrosine or engagement of the SH2 or SH3 domains by their ligands releases Lck from its inactive conformation, resulting in a primed, but not fully active, Lck kinase (see Fig. 7.12). Full activation of catalytic activity also requires Lck autophosphorylation of a tyrosine in its kinase domain. In nonstimulated lymphocytes, phosphorylation of Lck is counteracted by the tyrosine phosphatase CD45, which can dephosphorylate both of the Lck tyrosine phosphorylation sites. Prior to TCR stimulation, multiple phosphorylated species of Lck are found in T cells, but antigen receptor stimulation is required to stabilize the activated form of Lck and lead to ITAM phosphorylation.
Fig. 7.12 Lck activity is regulated by tyrosine phosphorylation and dephosphorylation. Src kinases such as Lck contain Sh3 (blue) and Sh2 (orange) domains preceding the kinase domain (green). Lck also contains a unique amino-terminal motif (yellow) with two cysteine residues that bind a Zn ion that is also bound to a similar motif in the cytoplasmic domain of cd4 or cd8. upper panel: in inactive Lck, the two lobes of the kinase domain are constrained by interactions with both the Sh2 and Sh3 domains. The Sh2 domain interacts with a phosphorylated tyrosine (red) at the carboxy-terminal end of the kinase domain. The Sh3 domain interacts with a proline-rich sequence in the linker between the Sh2 domain and the kinase domain. middle panel: dephosphorylation of the carboxyterminal tyrosine by the phosphatase cd45 (not shown) releases the Sh2 domain. Binding of other ligands to the Sh3 region can facilitate release of the linker region (not shown). in this state, Lck is considered primed, but not fully activated. Lower panel: full activation of Lck catalytic activity requires autophosphorylation on the activation loop in the kinase domain. active Lck can then phosphorylate iTams in the signaling chains of the nearby T-cell receptor. Rephosphorylation of the carboxy-terminal tyrosine by the c-terminal Src kinase (csk) or loss of the Sh3 ligand returns Lck to the inactive state.