The regulation of antigen-receptor signaling by protein tyrosine phosphatases: a hole in the story.

Antigen-receptor signaling requires Src-family kinases to initiate tyrosine phosphorylation. CD45 dephosphorylates the inhibitory site in Src-family kinases before antigen-receptor engagement and thus serves to 'prime' the kinases. It has been unclear why CD45 does not also dephosphorylate 'activated' kinases or motifs within the cytoplasmic domains of antigen-receptors and thus prevent signal transduction. Recent reports raise the possibility that CD45 is excluded from engaged antigen-receptors by mechanisms that may include the formation of lipid microdomains.