Homology of Ti α-subunit of a T-cell antigen–MHC receptor with immunoglobulin

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Abstract
Human T-cell receptors for antigen and major histocompatibility complex (MHC) determinants have now been defined on inducer1, suppressor2, and class 1 and class 2 MHC-specific cytotoxic T lymphocytes3 as T3-associated clonotypic molecules (Ti) of relative molecular mass 90,000 (90K) composed of one 49–54K α- and one 43K β-subunit which are disulphide-linked4. In the case of the Ti β-subunit, N-terminal amino acid sequencing5 and molecular cloning techniques6,7 led recently to identification of the Ti β-gene and showed that T-specific V, D, J and C segments fuse to form an active β-gene8,9. So far, however, there have been little structural data available on the Ti α-subunit. Here we have derived the amino acid sequence of a portion of the Ti α-subunit by CNBr fragmentation. Sequence analysis reveals ∼40% homology between the Ti α-subunit fragment and the third framework of the variable region of immunoglobulin light and heavy chains, supporting the notion that the Ti α-subunit is a member of the immunoglobulin–Ti β-gene family.