Molecular characterization of the murine cytotoxic T-cell membrane glycoprotein Ly-3 (CD8)

The murine Ly-2/3 glycoprotein is a surface marker of T cells restricted by class I major histocompatibility complex antigens. It is a disulfide-bonded heterodimer in which either the alpha or alpha' polypeptide chain encoded by Ly-2 is covalently linked to the beta polypeptide chain encoded by Ly-3. The nucleotide and predicted amino acid sequence of the murine Ly-3 cDNA, isolated by using the rat Ly-3 cDNA clone pX9.15, together with the amino acid sequence of Ly-3.1 peptides and the N terminus, are presented here. The alignment of peptide data from the Ly-3.1 antigen with that of the predicted amino acid sequence of the Ly-3.2 antigen confirmed that the putative Ly-3 cDNA clones do in fact encode the Ly-3 protein. The Ly-3.2 cDNA clones encode a protein of 213 amino acids, which includes a 21-residue leader sequence and structural features in common with immunoglobulin variable, joining, and hinge regions. Searches of protein data bases revealed that Ly-3 is a member of the immunoglobulin superfamily with significant homology to Ly-2, immunoglobulin variable region kappa and lambda light chains, and the beta chain of the T-cell receptor. A single N-linked glycosylation site was found at asparagine-13. The relative expression of two mRNA species (approximately 1.3 and 2.3 kilobases) varied according to the source of mRNA. A murine B1 repeat was located in the 3' untranslated region of Ly-3 cDNA clones.