Secondary, tertiary, and quaternary structure of T-cell-specific immunoglobulin-like polypeptide chains

To explore the possibility that the difference in antigen recognition between B and T cells derives from a structural difference in their respective antigen-specific receptors (immunoglobulins on B cells and immunoglobulin-like molecules on T cells), we compared the extracellular segments of the T-cell receptor α, β, and γ polypeptide chains and the N-terminal segment of the T-cell T8 (Lyt-2) antigen chain with the corresponding regions of immunoglobulins whose three-dimensional structures are known. The results indicate that the four T-cell polypeptide chains are organized into immunoglobulin-like domains consisting of multistranded antiparallel β-sheet bilayers. Invariant amino acid side chains that are conserved in diversed immunoglobulins, including those that mediate domain-domain interactions and form a constant scaffold for antibody binding sites, are also conserved in the chains encoded by the T-cell receptor genes and in the N-terminal domain of T8 (Lyt-2). It appears that the binding sites of the antigen-specific T-cell αβ-chain receptors and of antibodies are very similar in their overall dimensions and geometry: a T-cell αβ receptor molecule probably has an antigen-specific binding site that is fundamentally no different than the conventional binding site of an antibody.