Homology of Ti Alpha-subunit of a T-cell Antigen-MHC Receptor with Immunoglobulin

Human T-cell receptors for antigen and major histocompatibility complex (MHC) determinants have now been defined on inducer, suppressor, and class 1 and class 2 MHC-specific cytotoxic T lymphocytes as T3-associated clonotypic molecules (Ti) of relative molecular mass 90,000 (90K) composed of one 49-54K alpha- and one 43K beta-subunit which are disulphide-linked. In the case of the Ti beta-subunit, N-terminal amino acid sequencing and molecular cloning techniques led recently to identification of the Ti beta-gene and showed that T-specific V, D, J and C segments fuse to form an active beta-gene. So far, however, there have been little structural data available on the Ti alpha-subunit. Here we have derived the amino acid sequence of a portion of the Ti alpha-subunit by CNBr fragmentation. Sequence analysis reveals approximately 40% homology between the Ti alpha-subunit fragment and the third framework of the variable region of immunoglobulin light and heavy chains, supporting the notion that the Ti alpha-subunit is a member of the immunoglobulin-Ti beta-gene family.