Fig. P1.

CD45 differentially regulates Src family kinase phosphorylation in T and B cells. Increasing the expression of the signaling molecule CD45 leads to progressive dephosphorylation of the inhibitory tyrosine of the Src family kinases in both T and B cells. However, only in T cells does increasing CD45 expression result in dephosphorylation of the tyrosine in the activation loop of the T cell-specific Src family kinase (Lck). This, in turn, confers a dual positive and negative regulatory role on CD45 during T-cell antigen receptor signaling. In contrast, CD45 plays a purely positive regulatory role during B-cell antigen receptor signaling.