Structure of the Chicken CD3 εδ/γ Heterodimer and Its Assembly with the αβT Cell Receptor*

Background: Chickens possess a CD3/ chain that assembles with T cell receptor to mediate immune signaling. Results: Chicken CD3E/ has an atypical heterodimer interface and surface but associates with TCR. Conclusion: Chicken CD3/ represents a hybrid chain possessing features in common with human CD3 and CD3. Significance: Understanding the ancestral TCR signaling complex provides insights into the evolution of this signaling apparatus. In mammals, the T cell receptor (TCR) signaling complex is composed of a TCR heterodimer that is noncovalently coupled to three dimeric signaling molecules, CD3E, CD3E, and CD3. The nature of the TCR signaling complex and subunit arrangement in different species remains unclear however. Here we present a structural and biochemical analysis of the more primitive ancestral form of the TCR signaling complex found in chickens. In contrast to mammals, chickens do not express separate CD3 and CD3 chains but instead encode a single hybrid chain, termed CD3/, that is capable of pairing with CD3E. The NMR structure of the chicken CD3E/ heterodimer revealed a unique dimer interface that results in a heterodimer with considerable deviation from the distinct side-by-side architecture found in human and murine CD3E and CD3E. The chicken CD3E/ heterodimer also contains a unique molecular surface, with the vast majority of surface-exposed, nonconserved residues being clustered to a single face of the heterodimer. Using an in vitro biochemical assay, we demonstrate that CD3E/ can assemble with both chicken TCR and TCR via conserved polar transmembrane sites. Moreover, analogous to the human TCR signaling complex, the presence of two copies of CD3E/ is required for assembly. These data provide insight into the evolution of this critical receptor signaling apparatus.