The organizing principle in the formation of the T cell receptor-CD3 complex

Abstract

The T cell receptor (TCR) serves a critical function in the immune system and represents one of the most complex receptor structures. A striking feature is the presence of nine highly conserved, potentially charged residues in the transmembrane helices. Previous models have attempted to explain assembly based on pairwise interactions of these residues. Using a novel method for the isolation of intact radiolabeled protein complexes, we demonstrate that one basic and two acidic transmembrane residues are required for the assembly of each of the three signaling dimers with the TCR. This remarkable three-helix arrangement applies to all three assembly steps and represents the organizing principle for the formation of this intricate receptor structure.

Figure 1. The TCR-CD3 Complex Assembled in the ER Contains One TCRαβ Heterodimer

(A) Isolation of intact radiolabeled protein complexes by sequential non-denaturing immunoprecipitation (snIP). A conceptual illustration of denaturing (left side) versus non-denaturing (right side) sequential immunoprecipitation (IP) for analysis of non-covalent protein complexes. (B) Potentially charged TM residues in the TCR and associated signaling dimers. Three conserved basic residues are present in the TM domains of the TCR (arginine and lysine in TCRα—dark and light blue, respectively—and lysine in TCRβ), while six acidic residues are located in the TM domains of the CD3δε, CD3γε, and ζζ dimers. Note that the TCRα arginine and the two acidic residues of ζζ are located in the upper third of the putative TM domains, while the lysine residues of TCRα and β as well as the acidic residues of CD3δε and γε are located approximately in the center. (C and D) Comparison of single-step IP and two-step snIP analyses of in vitro assembled TCR-CD3 complexes. (C) A single translation/assembly reaction containing mRNAs encoding human TCRα, TCRβ tagged with a streptavidin binding peptide (TCRβ_SBP), and CD3 γ, δ, ε, and ζ chains was carried out as described in Experimental Procedures. The reaction was split 8 ways and cleared digitonin lysates were analyzed by single-step IP using antibodies specific for the indicated targets (lanes 1, 3, 4, 6, and 7) or control (c) reagents (lane 2 agarose beads; lane 5 control goat polyclonal; lane 8 control mIgG1 mAb). (D) Isolation of intact TCR-CD3 complexes by sequential non-denaturing IP (snIP). A single translation/assembly reaction was performed as in (C) and split 5 ways. Membrane fractions were either solubilized in SDS and run directly as loading controls (lane 1) or solubilized in digitonin and subjected to SA capture. Intact protein complexes were eluted from SA beads with free biotin and reprecipitated using antibodies to the indicated (2° IP) targets (lanes 3, 5, 7, and 9). In a control reaction (lanes 2, 4, 6, 8, and 10), the TCRβ mRNA did not encode the SBP affinity tag. Loading controls here and elsewhere represent approx. 10% of the starting material used for an IP. (E) The core TCR-CD3 complex contains a single TCRαβ heterodimer. Assembly reactions were carried out as in (C) and included either TCRβ_HA (lane 1), TCRβ_SBP (lane 2), or both (lanes 3–6). TCRαβ_HA and TCRαβ_SBP (arrows) could be resolved on a 12% SDS gel due to the size of the SBP tag (lanes 1–3). IP for one β chain coprecipitated associated CD3 polypeptides, but not the alternative β chain (lanes 4 and 5), and TCRβ_SBP→TCRβ_HA snIP failed to recover any labeled proteins (lane 6). In a positive control experiment (F), the SBP/HA tag pair was used to isolate complexes containing two different CD3ε chains. Formation of disulfide linked CD3ε dimers (†) is a competing reaction with CD3δε and CD3γε formation and represents a side-product. This band is marked with the same symbol where it appears in other experiments. Lane 7 is a control reaction (*) in which full complexes containing each tagged CD3ε chain were assembled separately and mixed prior to solubilization.

Figure 2. Interaction Among Three TM Domains that Requires One Basic and Two Acidic TM Residues

(A) A helical wheel diagram shows the relative positions of the two basic residues in the TCRα TM region. Position 1 (V, valine) represents the first predicted TM residue at the extracellular (or ER luminal) face of the membrane. A series of experiments was performed to examine the requirements for basic and acidic TM residues in the association of CD3δε with TCR. (B–D) analyze the four-chain TCRαβ-CD3δε complex, and (E and F) demonstrate that the same polar residues are relevant for the three-chain TCRα-CD3δε complex. CD3 heterodimers were isolated by CD3δ_PC→CD3ε snIP (B, C, E, and F) or single anti-CD3ε IP (D) (IP strategy indicated above each image), in order to determine the effect of mutations on assembly with TCR (arrows). In (B), the basic TM residues in the TCR chains were substituted in all possible combinations to assess their role in assembly with CD3δε (R-arginine, K-lysine, and A-alanine). The TCRα TM lysine is necessary and sufficient for this assembly step, as TCR association was lost only when this residue was substituted (boxes). The same result was obtained when TCRβ was omitted in the assembly reaction (E). (C and D) Both acidic TM residues in CD3δε are required for association with TCR. While substitution of either aspartic acid residue with alanine [D→A] severely reduced or eliminated TCR association, conservative substitution with asparagine [D→N] was tolerated to a significant degree in either CD3 chain, but not both. (D) The same result was obtained regardless of whether an epitope tag was used in the IP (C) or untagged complexes were precipitated with a mAb to the CD3ε extracellular domain (D). Control lanes (*) consisted of TCR and CD3 components assembled in separate reactions and mixed prior to solubilization. Quantitation of associated TCR proteins was done by densitometry using a phosphor imager, and values are expressed as percentages relative to levels of association observed among wild-type proteins for each experiment.

Figure 3. Highly Specific Requirements for TCRα-CD3δε TM Interactions

(A) TCRα-CD3δε association is driven by the TM domains. An ectodomain-truncated TCRα polypeptide consisting of only the TM region and 6 flanking N-terminal and five C-terminal residues as well as a C-terminal SBP tag (TCRα_TM–SBP) assembled with CD3δε (lane 1). Assembly was disrupted by D→A substitution in either CD3 chain (lanes 3, 5, and 7), as observed for full-length TCRα. (B–D) Stringent requirement for a specific arrangement of one basic and two acidic TM residues in TCRα-CD3δε assembly. Assembly was abrogated by substituting TCRα TM lysine [K] with any other polar residues (B), or by exchanging the position of the native lysine [K] or aspartic acid [D] residues among the three TM helices (C, 1–3). Alanine substitution at the position of any potentially charged TM residue also abrogated assembly (C, 4–6), and substitution at all three positions with asparagine did not support association (D). Posttranslation mixing controls (*) were performed as before, and loading controls are included for experiments in (A), (B), and (D). IP strategy is indicated above each experiment. (E) Illustration depicting CD3δε association with TCRα via the TM lysine [K]. TCR-CD3 components are represented as whole polypeptides (left) and as an axial view of simplified helical wheels (right).

Figure 4. TCRβ TM Lysine and Both CD3γε TM Acidic Residues Are Required for TCRβ-CD3γε Association

(A–C) TCRβ TM lysine [K] is required for CD3γε association with TCR. In reactions containing TCRα, β, and CD3γ, δ, and ε, alanine substitution of TCRβ TM K results in loss of TCR association with CD3γ_PCε (A) and specific loss of CD3γ from TCR-CD3 subcomplexes selected by a different IP strategy (B). The same result is obtained when the complexity is reduced to the three-chain TCRβ-CD3γε complex (C). (D) CD3γ associates with TCR more efficiently when CD3δ is also present. TCR-CD3 complexes were assembled in reactions containing TCRαβ_SBP and CD3δε (lane 1), CD3γε (lane 2), or CD3γδε (lane 3). TCR-CD3 products were isolated as in (B). Relative signal recovered in the indicated bands was quantitated by densitometry using a phosphor imager. (E) Both CD3γε TM acidic residues are required for association with TCR. Assembly reactions were performed and analyzed precisely as in (A), with conservative (aspartic acid to asparagine, D→N; glutamic acid to glutamine, E→Q) or non-conservative (alanine, A) substitutions in CD3γε TM regions. Quantitation and mixing controls (*) were performed as above.

Figure 5. Evidence that TCRα-CD3δε and TCRβ-CD3γε Interactions Are Similar in the Membrane

(A) Both CD3γ and δ are required for ζ-chain association. Reactions contained all TCR-CD3 components, as indicated below gels (lanes 1, 4, and 7), or singly lacked γ (lanes 2, 5, and 8), or δ (lanes 3, 6, and 9). No fully assembled complexes were recovered from reactions lacking either CD3γ or CD3δ (lanes 5 and 6; middle image), but TCR-CD3 sub-complexes not containing ζ chain were revealed by alternative snIP analysis of the same reactions (right image). Loading controls are provided as for previous experiments (left image). (B) TCRβ can associate with either CD3 heterodimer in three-chain assembly reactions. TCRα_PC or β_SBP were each cotranslated with CD3δε or γε and three-chain complexes were isolated by α_PC→ε (lanes 1 and 2) or β_SBP→ε (lanes 3 and 4) snIP. (C) TCRαβ-CD3δε complexes contain only one CD3δε heterodimer. A strategy similar to that employed in Figures 1E and 1F was used to assess whether two CD3δε dimers could associate with a single TCR. (D) CD3γε associates with TCR via the TCRβ TM lysine (K) in a TM helical interaction similar to that seen for TCRδ-CD3δε. Two views are provided as in Figure 3E.

Figure 6. The TCRα TM Arginine and Both ζ Chain TM Aspartic Acid Residues Are Important for ζζ Association

(A and B) TCRα TM arginine is important for ζ chain association. Full assembly reactions were analyzed using two different snIP strategies to examine effects of conservative and non-conservative substitutions. (A) Specific loss of ζζ homodimer (arrowhead) was observed when TCRα TM arginine was mutated to alanine [R→A] (lane 5). (B) Quantitation of fully assembled complexes revealed that the R→K substitution reduced ζζ association to less than half of wild-type levels (lane 3), while the R→A substitution reduced ζζ association to almost undetectable levels (lane 5). In control reactions (*), TCR-CD3 components and ζζ homodimers were assembled separately and mixed prior to solubilization. (C and D) Both ζ chain TM aspartic acid residues are important for ζζ association. (C) Disulfide-linked ζζ mixed dimers containing two different ζ chains in each reaction were isolated by ζ_PC→ζ_HA snIP and separated by non-reducing SDS-PAGE. Symbols indicate the wild-type dimer (DD), and mixed dimers in which the aspartic acid of one ζ chain was substituted with glutamic acid (DE), asparagine (DN), or lysine (DK). All combinations efficiently formed covalent mixed dimers. Application of this approach to full assembly reactions (D) revealed reduced ζζ assembly with TCR when one ζ chain carried a conservative substitution (DE, lane 3; DN, lane 5), and elimination of the interaction by a non-conservative substitution (DK, lane 7). In control reactions (*), full complexes with each ζ chain were assembled separately and mixed prior to solubilization.

Figure 7. Assembly of the TCR-CD3 Complex Is Organized by Three Major Assembly Steps in the Membrane, Each Involving One Basic and Two Acidic Residues

(A) Each of the three basic residues in the TCR TM regions serves as a critical contact for one of the three signaling dimers that associate with TCRαβ. TCR-CD3 polypeptides are shown in a representation of the lipid bi-layer (upper image). Lengths of TM domains are in appropriate proportion to extracellular Ig domains. Simplified helical wheel projection depicting interactions among TM domains in the same relative positions as above (lower image). (B) Three-helix association may be favorable when a lysine (or arginine) is at the contact interface. Although lysine can adopt many conformations, close apposition of helices in the lipid may be achieved more effectively in a three-helix assembly than at a two-helix interface due to the length of lysine side chain (C).