T cell antigen receptor-eta subunit. Low levels of expression and limited cross-species conservation.

The eta-subunit of the TCR is derived from an alternative splice product of the TCR-zeta gene. The eta-subunit has been extensively characterized in murine T cells, where up to 10% of TCR bear zeta-eta heterodimers rather than zeta-homodimers. In contrast to the significant levels of eta found in murine T cells, in human PBL an eta-like region is expressed spliced to upstream exons of the zeta-gene at no more than 0.25% the level of zeta-mRNA. Analysis of genomic DNA from five additional mammalian species demonstrated that eta-like sequences are highly conserved at a nucleic acid level. The protein sequences encoded in the eta-regions from various species ranged from 28 to 92 amino acids in length, with the first seven deduced amino acids of eta-being common to all species. Within three to five amino acids of this region, all species have five consecutive charged amino acids. Beyond this point, due to translation in different reading frames, there is no significant amino acid homology. The findings of low level of expression of eta-RNA, limited cross-species conservation on a protein level, and the lack of an established functional role for this alternative splice product raise questions as to the potential roles that this subunit may play in TCR function.