Purification and NH2-terminal amino acid sequencing of the β subunit of a human T-cell antigen receptor

To obtain information about the structural basis for T-cell antigen recognition, a T3-associated Ti receptor molecule was isolated from crude membranes of the REX human thymic tumor line and purified by affinity chromatography with an anti-clonotypic monoclonal antibody in conjunction with preparative gel electrophoresis. NH₂-terminal amino acid sequencing of the β subunit unambiguously identified the amino acids in positions 2-12. Comparative protein sequence analysis by computer search demonstrated that this Ti β sequence bore weak, but definite, homology to the first framework of the variable region of human λ light chain. Antisera to a synthetic peptide corresponding to positions 2-11 precipitated the denatured Ti β subunit from REX, thus confirming the above sequence. This information suggests that the Ti β subunit is distantly related to human immunoglobulin λ light chain and, moreover, should be of use in the molecular cloning of the Ti β gene.