Direct suppression of TCR-mediated activation of extracellular signal-regulated kinase by leukocyte protein tyrosine phosphatase, a tyrosine-specificphosphatase

Leukocyte protein tyrosine phosphatase (LC-PTP)/hemopoietic PTP is a human cytoplasmic PTP that is predominantly expressed in the hemopoietic cells. R ecently, it was reported that hemopoietic PTP inhibited TCR-mediated signal transduction, However, the precise mechanism of the inhibition was not ide ntified. Here we report that extracellular signal-regulated kinase (ERK) is the direct target of LC-PTP, LC-PTP dephosphorylated ERK2 in vitro. Expres sion of wild-type LC-PTP in 293T cells suppressed the phosphorylation of ER K2 by a mutant MEK1, which was constitutively active regardless of upstream activation signals, No suppression of the phosphorylation was observed by LC-PTPCS, a catalytically inactive mutant. In Jurkat cells, LC-PTP suppress ed the ERK and p38 mitogen-activated protein kinase cascades, LC-PTP and LC -PTPCS made complexes with ERK1, ERK2, and p38 alpha, but not with the gain -of-function sevenmaker ERK2 mutant (D321N), A small deletion (aa 1-46) in the N-terminal portion of LC-PTP or Arg to Ala substitutions at aa 41 and 4 2 resulted in the loss of ERK binding activity. These LC-PTP mutants reveal ed little inhibition of the ERK cascade activated by TCR cross-linking. On the other hand, the wild-type LC-PTP did not suppress the phosphorylation o f sevenmaker ERK2 mutant. Thus, the complex formation of LC-PTP with ERK is the essential mechanism for the suppression. Taken collectively, these res ults indicate that LC-PTP suppresses mitogen-activated protein kinase direc tly in vivo.