. "Authored: Rudd, C.E., de Bono, B, Garapati, P V, 2008-01-24 15:53:10"^^ . "Reviewed: Trowsdale, J, 2008-02-26 12:02:59"^^ . "Csk is a tyrosine kinase that phosphorylates the negative regulatory C-terminal tyrosine residue Y505 of Lck to maintain Lck in an inactive state. In resting T cells, Csk is targeted to lipid rafts through engagement of its SH2 domain with phosphotyrosine residue pY317 of PAG. PAG is expressed as a tyrosine phosphorylated protein in nonstimulated T-cells. This interaction of Csk and PAG allows activation of Csk and inhibition of Lck. Given that PAG-1 T cell knock out show a weak phenotype, some other protein may substitute in activating Csk."^^ . . . . . . . . "Interaction of Csk with PAG"^^ . . . .