The TCR, ζ-chain and CD3 molecules together comprise the TCR complex.
The CD3γ, CD3δ and CD3ε chains are highly related cell surface proteins of the immunoglobulin superfamily containing a single extracellular immunoglobulin domain.
The transmembrane region of the CD3 chains is negatively charged, a characteristic that allows these chains to associate with the positively charged TCR chains (TCRα and TCRβ).
The intracellular tails of the CD3 molecules contain a single conserved motif known as an immunoreceptor tyrosine-based activation motif or ITAM for short, which is essential for the signaling capacity of the TCR.
Phosphorylation of the ITAM on CD3 renders the CD3 chain capable of binding an enzyme called ZAP70 (zeta associated protein), a kinase that is important in the signaling cascade of the T cell.
References
Cellular and Molecular Immunology (5th Ed.) Abbas AK, and Lichtman, Editor: Saunders, Philadelphia, 2003.
