N-terminal negatively charged residues in CD3ɛ chains as a phylogenetically conserved trait potentially yielding isoforms with different isoelectric points: Analysis of human CD3ɛ chains

CD3ɛ chains are essential to the structure, expression and signaling of T cell receptors. Here, we extend to human CD3ɛ our previous data in mouse CD3ɛ showing that, in T cells, proteolytic processing of the acidic N-terminal sequence of CD3ɛ chains generate distinct polypeptide species that can be identified by two-dimension (IEF-SDS PAGE) electrophoresis and immunoblot. This was shown first by showing the processing of a fusion protein of GFP and the extracellular domain of mouse CD3ɛ (mCD3GFP) expressed in Jurkat cells. Secondly, pI heterogeneity was also found in human CD3ɛ chains immunoprecipitated from the surface of Jurkat cells or PHA blasts of human blood T lymphocytes.Comparison of CD3ɛ chains from 27 different species shows that their N-terminal sequences share a strong acidic nature, despite the large differences in terms of length and composition, even among closely related species. Our results suggest that generation of CD3ɛ chain isoforms with different N-terminal sequence and pI is a general phenomenon. Thus, as previously observed in the mouse, the relative abundance of CD3ɛ chain species might regulate TCR/CD3 structure and function, including the strength of the interactions between CD3 dimers and the TCR clonotypic receptors, as well as TCR/CD3 activation thresholds.Interestingly, CD3ɛ chains from 7 out of 27 species studied have putative N-glycosylation (NxS or NxT) motifs in their Ig extracellular domain. Their location, plus the conservation of residues involved in domain organization, the interactions with other CD3 chains, or the TCR, and signal triggering add new data useful to establish a permissive topology for the interaction between CD3 dimers and the TCR chains.