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The cytoplasmic domain of the T cell receptor ζ subunit (ζ cyt) is sufficient to couple receptor ligation to intracellular signaling cascades, but little is known about its structure or mechanism of signaling. In aqueous solution, ζ cyt is unstructured. Here we report that in the presence of lipid vesicles ζ cyt assumes a folded structure. The folding transition is reversible and dependent on the presence of acidic phospholipids. In the lipid-bound conformation, ζ cyt is refractory to phosphorylation by src family tyrosine kinases, which are believed to play a key role in signal initiation in vivo. In the lipid-free, unstructured form, ζ cyt is readily phosphorylated, and phospho-ζ cyt exhibits neither membrane association nor structure induction. The conformational change may provide a mechanism for coupling receptor clustering to cytoplasmic signaling events.