Lyn, one of the Src family members, is predominantly expressed in hematopoietic cells. Two tyrosine residues have been reported to play a crucial role in the regulation of protein tyrosine kinases of the Src family. Autophosphorylation of Tyr396, located in the catalytic domain, correlates with enzyme activation. Csk-mediated phosphorylation of the carboxy-terminal Tyr507 inactivates the kinase. Tyrosine phosphorylation and activation of Lyn occurs upon association with cell surface receptors such as the B cell Ag receptor and CD40. Studies using knockout mice have shown that the net effect of Lyn deficiency is to render B cells hypersensitive to BCR stimulation, suggesting that the most critical role for Lyn in vivo is in the down-regulation of B cell responses. Lyn is also involved in controlling the migration and development of specific B cell populations.
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