STRUCTURAL-ANALYSIS OF THE CD3-ZETA-ETA LOCUS OF THE RAT - EXPRESSIONOF ZETA BUT NOT ETA-TRANSCRIPTS BY RAT T-CELLS

We analyzed the structure and pattern of expression of rat TCR zeta- a nd eta-chains to investigate if these components function in activatio n and development of rat T cells. The rat zeta cDNA contained the comp lete open reading frame coding for a polypeptide of 164 amino acids an d the 5' and 3' noncoding sequences. Comparison of the amino acid sequ ence to those of mouse and human counterparts revealed a high degree o f similarity, more than 85% homology among all three species except fo r the signal peptide, which was especially high in the cytoplasmic dom ain including the nucleotide binding site and the possible tyrosine ph osphorylation sites. Furthermore, we determined the nucleotide sequenc es of a rat genomic eta-like sequence located in the 3' region of the rat zeta-gene. Although it showed a high level of nucleotide similarit y to mouse and human counterparts, 90.4 and 78.9%, respectively, the d educed polypeptide was very short (only 28 residues) and markedly dive rgent from the mouse and human eta-specific polypeptides due to frames hift mutations. Transcription of rat zeta was shown to be highly restr icted to T cells; abundantly in thymocytes and scarcely in peripheral T cells. Surprisingly, the rat eta transcript could not be detected in any rat tissues so far tested by Northern blot analysis and even by t he sensitive reverse transcription-polymerase chain reaction method, w hereas it was readily detected in mouse thymus. These findings suggest that the zeta-chain has conserved roles in TCR assembly and TCR-media ted signaling. However, the eta-chain seems not to be indispensable be cause of its structural diversity among these three species characteri zed to date and the apparent lack of eta expression in the rat.