T cell antigen receptor (TCR)CD3 complex is composed of six different subunits: TCR and TCR and CD3, CD3, CD3, and CD3. 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 CD3 with a probe of glutathione S-transferase fused to the cytoplasmic region of CD3. One of these clones was found to encode topoisomerase II (topoII). The binding region of CD3 is located within the N-terminal 12 amino acids containing the motif of a basic amino acid cluster. A similar motif was found in the chain of Fc receptors (FcR) but not in the CD3 chain, and indeed, FcR but not CD3 bound to topoII. The binding region of topoII was determined to be the C terminus. Since this region appears to be the regulatory region of the enzymatic activity, the binding of CD3 might affect the function of topoII. Although topoII is localized mainly in the nucleus and CD3 is a membrane protein, we demonstrated the presence of CD3 in the nuclear fraction of thymocytes, which increased upon T cell activation. The specific interaction in cells was evidenced by co-immunoprecipitation of topoII and CD3 from the nuclear fraction of T cells. The possible function of this interaction is discussed.

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