Abstract
The T4 antigens of mouse and sheep have been purified to homogeneity and partially sequenced using protein and peptide microsequencing techniques. Both mouse and sheep antigens bear distinct homology to human T4, having an amino-terminal segment that is homologous to the K-chain variable region (VK). A surprisingly high degree of sequence divergence was nevertheless evident between the T4 antigens of the three mammalian species, suggesting an unusually rapid rate of evolution that is possibly related to the functional role of T4 in class II major histocompatibility complex antigen recognition. The mouse and sheep T4 antigens contain at least three intrachain disulfide bonds which in all cases connect adjacent cysteine residues. All three intrachain disulfide linkages are situated within the putative extracellular domain, and the amino-terminal disulfide bond probably involves the two cysteine residues homologous to those which form the intrachain bridge within the VK domain of immunoglobulin molecules. The structural relationship of the T4 antigen to other members of the immunoglobulin supergene family is discussed.