Structural analysis of the CD3 ζ/η locus of the rat: Expression of ζ but not η transcripts by rat T cells

We analyzed the structure and pattern of expression of rat TCR ζ- and η-chains to investigate if these components function in activation and development of rat T cells. The rat ζ cDNA contained the complete open reading frame coding for a polypeptide of 164 amino acids and the 5′ and 3′ noncoding sequences. Comparison of the amino acid sequence to those of mouse and human counterparts revealed a high degree of similarity, more than 85% homology among all three species except for the signal peptide, which was especially high in the cytoplasmic domain including the nucleotide binding site and the possible tyrosine phosphorylation sites. Furthermore, we determined the nucleotide sequences of a rat genomic η-like sequence located in the 3′ region of the rat ζ-gene. Although it showed a high level of nucleotide similarity to mouse and human counterparts, 90.4 and 78.9%, respectively, the deduced polypeptide was very short (only 28 residues) and markedly divergent from the mouse and human η-specific polypeptides due to frameshift mutations. Transcription of rat ζ was shown to be highly restricted to T cells; abundantly in thymocytes and scarcely in peripheral T cells. Surprisingly, the rat η transcript could not be detected in any rat tissues so far tested by Northern blot analysis and even by the sensitive reverse transcription-polymerase chain reaction method, whereas it was readily detected in mouse thymus. These findings suggest that the ζ-chain has conserved roles in TCR assembly and TCR-mediated signaling. However, the η-chain seems not to be indispensable because of its structural diversity among these three species characterized to date and the apparent lack of η expression in the rat.