The resting and ligand-bound states of the membrane-embedded human T-cell receptor–CD3 complex

The T-cell receptor (TCR) initiates T-lymphocyte activation, but mechanistic questions remain( 1–4 ). Here, we present cryogenic electron microscopy structures for the unliganded and human leukocyte antigen (HLA)-bound human TCR–CD3 complex in nanodiscs that provide a native-like lipid environment. Distinct from the “open and extended” conformation seen in detergent( 5–8 ), the unliganded TCR–CD3 in nanodiscs adopts two related “closed and compacted” conformations that represent its physiologic resting state in vivo . By contrast, the HLA-bound complex adopts the open and extended conformation, and conformation-locking disulfide mutants show that ectodomain opening is necessary for maximal ligand-dependent T-cell activation. Together, these results reveal allosteric conformational change during TCR activation and highlight the importance of native-like lipid environments for membrane protein structure determination.