BIOCHEMICAL-CHARACTERIZATION OF THE ETA-CHAIN OF THE T-CELL RECEPTOR - A UNIQUE SUBUNIT RELATED TO ZETA

  • 5 September 1989
    • journal article
    • research article
    • Vol. 264  (25) , 14812-14817
Abstract
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 group of components of the T-cell receptor.