CD3ε homologues in the chondrostean fish Acipenser ruthenus

Abstract.

CD3ε is an essential component of the T-cell receptor (TCR) complex for antigen. We report here molecular cloning and characterization of cDNAs encoding the CD3ε homologues in sterlet (Acipenser ruthenus), a representative of primitive chondrostean fishes. Sequence analysis of the cDNA clones demonstrated unexpectedly high CD3 ε gene heterogeneity in this species. While some cDNAs encoded proteins with the structure typical of mammalian CD3ε, others coded for proteins lacking the membrane-proximal half of the extracellular domain. Two cDNAs contained in-frame stop codons in the region encoding the cytoplasmic domain. Based on genomic blot analysis and RT-PCR typing of individual spleen RNAs, we suggest that sterlet may possess two highly polymorphic CD3 ε loci, of which one can produce alternatively spliced transcripts. The structural elements shown to be functionally important in the mammalian CD3ε are strongly conserved in the sterlet CD3ε. The cytoplasmic region contains an immunoreceptor tyrosine-based activation motif (ITAM) with YEPI and YSGL tyrosine-containing sequences that are characteristic of only this TCR subunit. The pattern of sequence conservation indicates also that strong selection pressure was imposed on a motif VYYW at the C-end of the transmembrane domain and on a CD3ε-specific proline-rich motif RXPPVP juxtaposed to the N-terminus of the ITAM. Weak similarity of the sterlet CD3ε with the chicken and Xenopus CD3γ/δ indicates that these two TCR subunits diverged before radiation of bony fishes and tetrapods. While the role of CD3ε heterogeneity in sterlet remains to be elucidated, the data obtained show that the basic mechanisms of TCR signaling have ancient evolutionary origin.