CAST, a novel CD3 epsilon-binding protein transducing activation signal for interleukin-2 production in T cells

Antigen recognition through T cell receptor (TCR)CD3 complex transduces sig nals into T cells, which regulate activation, function, and differentiation of T cells. The TCR-CD3 complex is composed of two signaling modules repre sented by CD3 zeta and CD3 epsilon. Signaling through CD3 zeta has been ext ensively analyzed, but that via CD3 epsilon, which is also crucial in immat ure thymocyte development, is still not clearly understood. We isolated cDN A encoding a novel CD3 epsilon-binding protein CAST. CAST specifically inte racts in vivo and in vitro with CD3 epsilon but not with CD3 zeta or FcR ga mma via a unique membrane-proximal region of CD3 epsilon. CAST is composed of 512 amino acids including a single tyrosine and undergoes tyrosine phosp horylation upon TCR stimulation. Overexpression of two dominant-negative ty pes of CAST, a minimum CD3 epsilon-binding domain and a tyrosine-mutant, st rongly suppressed NFAT activation and interleukin-2 production. These resul ts demonstrate that CAST serves as a component of preformed TCR complex and transduces activation signals upon TCR stimulation and represents a new si gnaling pathway via the CD3 epsilon-containing TCR signaling module.