Specific interaction of topoisomerase IIβ and the CD3ε chain of the T cell receptor complex

Abstract

T cell antigen receptor (TCR) · CD3 complex is composed of six different subunits: TCRα and TCRβ and CD3γ, CD3δ, CD3ε, and CD3ζ. Antigen recognition signals are transduced from TCR to the cytoplasm through the cytoplasmic domain of the CD3 chains. To understand the downstream signal transduction pathways, we cloned genes encoding proteins capable of binding to CD3ε with a probe of glutathione S-transferase fused to the cytoplasmic region of CD3ε. One of these clones was found to encode topoisomerase IIβ (topoIIβ). The binding region of CD3ε is located within the N-terminal 12 amine acids containing the motif of a basic amine acid cluster. A similar motif was found in the γ chain of Fc receptors (FcRγ) but not in the CD3ζ chain, and indeed, FcRγ but not CD3ζ bound to topoIIβ. The binding region of topoIIβ was determined to be the C terminus. Since this region appears to be the regulatory region of the enzymatic activity, the binding of CD3ε might affect the function of topoIIβ. Although topoIIβ is localized mainly in the nucleus and CD3ε is a membrane protein, we demonstrated the presence of CD3ε in the nuclear fraction of thymocytes, which increased upon T cell activation. The specific interaction in cells was evidenced by co- immunoprecipitation of topoIIβ and CD3ε from the nuclear fraction of T cells. The possible function of this interaction is discussed.