CD45 MODULATES PHOSPHORYLATION OF BOTH AUTOPHOSPHORYLATION AND NEGATIVE REGULATORY TYROSINES OF LYN IN B-CELLS

CD45 is a tyrosine phosphatase that is required for normal B cell rece ptor (BCR)-mediated signaling. It has been shown that Src-family tyros ine kinases such as Lyn could be a potential substrate for CD45. In vi tro studies indicate that activities of Src family tyrosine kinases ar e regulated by tyrosine phosphorylation; C-terminal phosphorylation is inhibitory, and autophosphorylation is stimulatory. We report here th at both autophosphorylation and C-terminal negative regulatory tyrosin es of Lyn were hyperphosphorylated in CD45-deficient DT40 B cells, In this mutant cell, BCR-induced protein-tyrosine phosphorylation and cal cium mobilization were severely compromised, as seen in Lyn-deficient cells. Consistent with this observation, Lyn activation upon receptor ligation was profoundly decreased in CD45-deficient cells. Taken toget her, our results suggest that dephosphorylation of tyrosine residues a t both autophosphorylation and negative regulatory sites is mediated b y CD45 in vivo, and that dephosphorylation of C-terminal tyrosine is a prerequisite for participation of Lyn in BCR signaling.