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The CD45 tyrosine phosphatase has been reported to activate the src family tyrosine kinases Lck and Fyn by dephosphorylating regulatory COOH-terminal tyrosine residues 505 and 528, respectively. However, recent studies with CD45- T-cell lines have found that despite the fact that Lck and Fyn were constitutively hyperphosphorylated, the tyrosine kinase activity of both enzymes was actually increased. In the present study, phosphoamino acid analysis revealed that the increased phosphorylation of Lck in CD45- YAC-1 T cells was restricted to tyrosine residues. To understand the relationship between tyrosine phosphorylation and Lck kinase activity, CD45- YAC-1 cells were transfected with forms of Lck in which tyrosines whose phosphorylation is thought to regulate enzyme activity (Tyr-192, Tyr-394, Tyr-505, or both Tyr-394 and Tyr-505) were replaced with phenylalanine. While the Y-to-F mutation at position 192 (192-Y--&gt;F) had little effect, the 505-Y--&gt;F mutation increased enzymatic activity. In contrast, the 394-Y--&gt;F mutation decreased the kinase activity to very low levels, an effect that the double mutation, 394-Y--&gt;F and 505Y--&gt;F, could not reverse. Phosphopeptide analysis of tryptic digests of Lck from CD45- YAC-1 cells revealed that it is hyperphosphorylated on two tyrosine residues, Tyr-505 and, to a lesser extent, Tyr-394. The purified and enzymatically active intracellular portion of CD45 dephosphorylated Lck Tyr-394 in vitro. These results demonstrate that in addition to Tyr-505, CD45 can dephosphorylate Tyr-394, and that in the absence of CD45 the hyperphosphorylation of Tyr-394 can cause an increase in the kinase activity of Lck despite the inhibitory hyperphosphorylation of Tyr-505. Therefore, Lck kinase activity is determined by the balance of activating and inhibitory tyrosine phosphorylations that are, in turn, regulated by CD45.

Mutational Analysis of Lck in CD45-Negative T Cells: Dominant Role of Tyrosine 394 Phosphorylation in Kinase Activity