Pp56LckMediates TCR ζ-Chain Binding to the Microfilament Cytoskeleton

Abstract

AbstractThe TCR ζ-chain (ζ) on mature murine T lymphocytes binds to the microfilament cytoskeleton in response to Ag receptor ligation. Here, we report the role of Src family kinases in ζ-cytoskeletal binding, using mutant mice and a cell-free model system. Binding of ζ to actin in the cell-free system has a specific requirement for ATP and divalent cations, with an apparent Michaelis-Menton constant for ATP in the millimolar range, and can be disrupted by either EDTA or the microfilament poison, cytochalasin D, suggesting that microfilaments provide the structural framework for an active process involving cellular kinases. Indeed, tyrosine-phosphorylated ζ is a predominant form of the ζ-chain bound to polymerized actin, while challenge with alkaline phosphatase prevents ζ-chain association in solution and releases ζ-chain from the bound state. Phosphorylated Src-family kinase pp56Lck also associates with membrane skeleton upon TCR engagement and is a component of the reconstituted cytoskeletal pellet. ζ-Chain phosphorylation and ζ-cytoskeletal binding are abrogated in cell lysates with reduced levels of pp56Lck and in activated mutant murine T cells lacking pp56Lck, implicating pp56Lck as the kinase involved in ζ-chain tyrosine phosphorylation and ζ-cytoskeletal binding. Finally, recombinant Lck Src homology 2 domain preferentially inhibits reconstituted ζ-cytoskeleton association, suggesting that ζ-microfilament binding is dependent on interactions between phosphorylated tyrosine residues in ζ-chain activation motifs and the Src homology 2 domain of the Lck protein tyrosine kinase.