Cutting edge: the CD45 tyrosine phosphatase is an inhibitor of Lck activity in thymocytes

J Immunol. 1999 Feb 15;162(4):1879-83.


A widely accepted model for regulation of the Lck tyrosine kinase is that it is activated by CD45-mediated dephosphorylation of its COOH-terminal negative regulatory tyrosine (Tyr505). Previous work from our laboratory, however, found that despite hyperphosphorylation of Tyr505, the activity of Lck from CD45- T cell lines was actually increased due to hyperphosphorylation of the positive regulatory tyrosine, residue 394. To avoid potential complications introduced by transformed cells, in this study we have characterized the effect of CD45 on Lck activity in normal cells. Lck in thymocytes from CD45-/- mice was hyperphosphorylated on tyrosine residues. Importantly, and in disagreement with the model that CD45 only activates Lck in vivo, the kinase activity of Lck from cells lacking CD45 was substantially increased. These results support a model in which CD45 dephosphorylates both Tyr505 and Tyr394, the net effect in normal thymocytes being a decrease in enzymatic activity.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Enzyme Activation / immunology
  • Enzyme Inhibitors / pharmacology*
  • Flow Cytometry
  • Leukocyte Common Antigens / genetics
  • Leukocyte Common Antigens / physiology*
  • Lymphocyte Specific Protein Tyrosine Kinase p56(lck) / antagonists & inhibitors*
  • Lymphocyte Specific Protein Tyrosine Kinase p56(lck) / metabolism
  • Mice
  • Mice, Inbred C57BL
  • Mice, Knockout
  • Molecular Sequence Data
  • Phosphorylation
  • Proto-Oncogene Proteins / metabolism
  • Proto-Oncogene Proteins c-fyn
  • T-Lymphocytes / enzymology*
  • T-Lymphocytes / immunology
  • T-Lymphocytes / metabolism
  • Thymus Gland / cytology
  • Thymus Gland / enzymology*
  • Thymus Gland / immunology
  • Tyrosine / metabolism


  • Enzyme Inhibitors
  • Proto-Oncogene Proteins
  • Tyrosine
  • Fyn protein, mouse
  • Lymphocyte Specific Protein Tyrosine Kinase p56(lck)
  • Proto-Oncogene Proteins c-fyn
  • Leukocyte Common Antigens