Molecular cloning of the CD3 eta subunit identifies a CD3 zeta-related product in thymus-derived cells.

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Abstract
The CD3.eta. subunit of the T-cell antigen receptor forms a heterodimeric structure with the CD3.zeta. subunit in thymus-derived lymphoid cells and is apparently involved in signal transduction through the receptor. Here we report the primary structure of murine CD3.zeta. as deduced from protein microsequencing and cDNA cloning. The mature protein is divided into three domains: a 9-amino acid extracellular segment, a 21-amino acid transmembrane segment including a negatively charged residue characteristic of CD3 subunits, and a 155-amino acid cytoplasmic tail. The NH2-terminal sequences of CD3.eta. and CD3.zeta. are identical through amino acid 122 of each mature protein but then diverge in the remainder of their respective COOH-terminal regions, consistent with alternatively spliced products of a common gene. The cytoplasmic domain of CD3.eta. is 42 amino acids larger than that of CD3.zeta. but lacks one of sic potential tyrosine phosphorylation sites as well as a putative nucleotide binding site previously identified in CD3.zeta.. These structural features presumably account for the difference between CD3.eta. and CD3.zeta. function and are consistent with the notion that CD3.eta. may be an important component of a T-cell receptor isoform(s) during thymic development.