Structure of the T-cell antigen receptor: evidence for two CD3 epsilon subunits in the T-cell receptor-CD3 complex.

Abstract

The T-cell antigen receptor (TCR) consists of heterodimeric glycoproteins (TCR a,4 or yS) that demonstrate homology with immunoglobulins. Noncovalently associated with the afi (or y8) heterodimer are at least five nonvariant proteins (CD3-y, -8, -e, -j, and -ig), which together comprise the TCR-CD3 complex. The stoichiometry of the antigen receptor has been assumed to be either apyBvSiCor a(y8e{7g.In this paper we provide several lines of evidence that support the notion that the mature TCR-CD3 complex on the cell surface contains two CD3-E polypeptide chains. Transfection of two murine T cell-T cell hybridomas with the human DNA encoding CD3-e protein demonstrated that both murine and human CD3-e chains were present within the same TCR-CD3 complex. Analysis of thymocytes isolated from transgenic mice that expressed high copy numbers of the human CD3-e gene showed that the heterologous human CD3-e subunits were coexpressed with murine CD3-E in the same TCR-CD3 complex. Since CD3-e was shown to form disulfide-linked homodimers both in human and murine T cells, the two CD3-e subunits present in the TCR-CD3 complex were in direct contact with one another. The presence of two CD3-E polypeptide chains in close proximity to one another in the TCR-CD3 complex may have important implications for its assembly and its signal transduction mechanisms.The T-cell antigen receptor (TCR) on thymus-derived lymphocytes (T cells), which recognizes antigen in association with either class I or class II major histocompatibility complex (MHC) products, consists of a heterodimer of glycoproteins, the a and /3 chain (1). A second form of the TCR, the y3 receptor, also exists on a smaller fraction of peripheral T cells (2). The TCR heterodimers are noncovalently associated with at least five nonvariant polypeptides: CD3-y, -8, -E, -;, and -'r (3, 4). CD3-; exists either as a disulfide-linked homodimer or, in the minority of TCR-CD3 complexes, as a disulfide-linked heterodimer with the CD3-i1 chain (4). The CD3 proteins appear to play an important role in transport of the TCR heterodimer to the cell surface and signal transduction after the TCR binds its appropriate ligand. The specific number of polypeptide chains that compose the TCR-CD3 complex on the cell surface has not been precisely determined. The stoichiometry has generally been assumed to be either a/3y6e4 or a/3^y&e'>r (3,4). In this paper, we analyze the stoichiometry of CD3-E in the complex by expression of human CD3-e (CD3-eh) in murine T cells. We provide evidence that there are two CD3-E polypeptide chains in close proximity to one another within the mature TCR-CD3 complex expressed on the cell surface.

Stoichiometry of Signalling Complexes in Immune Cells: Regulation by the Numbers

“…The results indicated that a TCR -TCR heterodimer binds a single CD3 -CD3 heterodimer, a single CD3 -CD3 heterodimer and a CD3 -CD3 homodimer. Thus, the overall stoichiometry of the TCR complex was found to be 1:1:1:1:2:2 (for TCR, TCR, CD3 , CD3 , CD3 , and CD3 , respectively) (Rudolph et al, 2006;Wucherpfennig et al, 2010) These findings corroborated the stoichiometry proposed in a previous study (Blumberg et al, 1990), and are supported by other results, as well (Call et al, 2004). However, other works suggested that a second heterodimer was incorporated into the complex, suggesting an ( ) 2 stoichiometry (Exley et al, 1995;Fernandez-Miguel et al, 1999;San Jose et al, 1998).…”

Structural Biology of the T-cell Receptor: Insights into Receptor Assembly, Ligand Recognition, and Initiation of Signaling

“…On ligand binding by the TCR, cytoplasmic ITAM motifs in the CD3 and z chains become phosphorylated by the Src-family kinase Lck (Weiss and Littman 1994), constituting the earliest detectable biochemical consequence of TCR ligation. Early genetic, biochemical, and immunofluorescencebased experiments established that the intact TCR-CD3 complex contains two copies of CD31 (Blumberg et al 1990b;de la Hera et al 1991), consistent with a model in which both CD3d1 and CD3g1 are incorporated into each abTCR complex. Similar approaches have failed to detect more than one TCRa or TCRb chain in intact TCR-CD3 complexes (Punt et al 1994;Call et al 2002), indicating that the receptor is monovalent with respect to the ligand-binding module.…”

The T cell receptor/CD3 complex is composed of at least two autonomous transduction modules

“…The remaining subunits, termed CD3-y, -ö, -E, -~, and -1], are invariant, noncovalently associated with the TCR aß dimer, and possess large intracytoplasmic domains thought to be responsible for coupling antigen recognition to various signal transductlon pathways. The evolutionarily related y, Ö, and e subunits are expressed as noncovalently associated ye and OE pairs (Koning et al, 1990;Blumberg et al , 1990;De la Herra et al , 1991), and display immunoglobulin-like extracellular domains (Gold et al, 1987). In contrast, the ~ and 1] subunlts contaln an extracellular domain of only 9 residues and constitute the prototype of a new protein family that includes the y chain of the high affinity IgE receptor (FceRI) (Weissmanetal., 1988;Jinetal., 1990;Küsteretal., 1990).…”