CAST, a Novel CD3ε-binding Protein Transducing Activation Signal for Interleukin-2 Production in T Cells

Antigen recognition through T cell receptor (TCR)-CD3 complex transduces signals into T cells, which regulate activation, function, and differentiation of T cells. The TCR-CD3 complex is composed of two signaling modules represented by CD3ζ and CD3ε. Signaling through CD3ζ has been extensively analyzed, but that via CD3ε, which is also crucial in immature thymocyte development, is still not clearly understood. We isolated cDNA encoding a novel CD3ε-binding protein CAST. CAST specifically interacts in vivo and in vitro with CD3ε but not with CD3ζ or FcRγ via a unique membrane-proximal region of CD3ε. CAST is composed of 512 amino acids including a single tyrosine and undergoes tyrosine phosphorylation upon TCR stimulation. Overexpression of two dominant-negative types of CAST, a minimum CD3ε-binding domain and a tyrosine-mutant, strongly suppressed NFAT activation and interleukin-2 production. These results demonstrate that CAST serves as a component of preformed TCR complex and transduces activation signals upon TCR stimulation and represents a new signaling pathway via the CD3ε-containing TCR signaling module.