Biochemical characterization of the eta chain of the T-cell receptor. A unique subunit related to zeta.

The T-cell antigen receptor is a multisubunit complex consisting of at least seven chains. Based upon structural and genetic considerations, we have divided these chains into three groups. The alpha and beta subunits (Ti) are the clonotypic chains responsible for antigen recognition. Three chains that are invariant among all T-cells define the CD3 complex. These include the CD3 gamma, delta, and epsilon chains. The zeta chain is a distinct component that, like the CD3 chains, is invariant among all T-cells. In the majority of receptors, zeta is found as a disulfide-linked homodimer. We have recently shown that approximately 10% of zeta is disulfide-linked to a chain which we have called eta. A preliminary model has been proposed, suggesting that there are two subclasses of receptors, depending upon the presence within the complex of either the zeta-zeta homodimer or the zeta-eta heterodimer. Evidence has been presented that these two subclasses may perform distinct signaling functions. In this paper the eta chain is characterized to determine whether it is structurally related to the zeta chain and, in particular, whether it might represent a post-translational modification of zeta. We can identify specific antigenic epitopes that are shared by both zeta and eta. However, not all antibodies raised against zeta can directly recognize eta. The apparent molecular mass of eta is 22 kDa, whereas zeta has a molecular mass of 16 kDa. We are unable to demonstrate any post-translational covalent modifications of eta to explain the difference in apparent molecular weight. These include phosphorylation, glycosylation, or sulfation. Amino acid incorporation studies demonstrate that the amino acid composition of eta is distinct from that of zeta. All of the eta in a T-cell is found in association with the rest of the components of the T-cell receptor. In addition, our anti-eta antibodies allow us to directly recognize human eta, which has an apparent molecular mass of approximately 23 kDa. Thus, eta and zeta appear to be related but distinct proteins, and we would propose that eta is the second member of the zeta group of components of the T-cell receptor.