Abstract
The catalytic activity of p56lck is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue (tyrosine 505). Accumulating data show that this phosphorylation is mediated by the tyrosine protein kinase p50csk and that it is reversed by the transmembrane tyrosine protein phosphatase CD45. Recent studies have indicated that dephosphorylation of tyrosine 505 in resting T cells is necessary for the initiation of antigen-induced T-cell activation. To better understand this phenomenon, we have characterized the factors regulating tyrosine 505 phosphorylation in an antigen-specific T-cell line (BI-141). As is the case for other T-cell lines, Lck molecules from unstimulated BI-141 cells exhibited a pronounced dephosphorylation of the inhibitory carboxyl-terminal tyrosine. This state could be corrected by incubation of cells with the tyrosine protein phosphatase inhibitor pervanadate, suggesting that it reflected the unrestricted action of tyrosine protein phosphatases. In structure-function analyses, mutation of the site of Lck myristylation (glycine 2) partially restored phosphorylation at tyrosine 505 in BI-141 cells. Since the myristylation-defective mutant also failed to stably associate with cellular membranes, this effect was most probably the consequence of removal of p56lck from the vicinity of membrane phosphatases like CD45. Deletion of the unique domain of Lck, or its replacement by the equivalent sequence from p59fyn, also increased the extent of tyrosine 505 phosphorylation in vivo. This effect was unrelated to changes in Lck membrane association and therefore was potentially related to defects in crucial protein-protein interactions at the membrane. In contrast, deletion of the SH3 or SH2 domain, or mutation of the phosphotransfer motif (lysine 273) or the site of autophosphorylation (tyrosine 394), had no impact on phosphate occupancy at tyrosine 505. In combination, these results indicated that the hypophosphorylation of the inhibitory tyrosine of p56(lck) in T lymphocytes is likely the result of the predominant action of tyrosine protein phosphatases. Moreover, they showed that both the amino-terminal myristylation signal and the unique domain of p56(lck) play critical roles in this process.