To obtain phases, crystals of σ1 receptor bound to PD144418 were grown using a hanging-drop LCP methodology adapted from a previous report32 (link). In brief, this entailed dispensing cubic phase drops onto a plastic cover film (Art Robbins Instruments) and overlaying with precipitant solution as described above. This film was then inverted over a matched plate with identical crystallization solutions to the precipitant surrounding the lipid drop. The resulting crystals could be soaked and resealed, unlike conventional glass sandwich lipidic cubic phase plates. Crystals prepared in this way were soaked with tantalum bromide clusters for approximately 12 hours by adding crushed granules of tantalum clusters to the edge of the well. The crystals were harvested and data collected as described above, but at a wavelength of 1.2548 Å.
Initial phases were obtained in SHARP33 using single isomorphous replacement and anomalous scattering (SIRAS). Three transmembrane α-helices were identifiable in the initial map, suggesting three molecules in the asymmetric unit with an unusual solvent content of ~70%. Experimental phases were iteratively combined with model-derived phase to improve the electron density map through solvent flattening in SHARP. Model building was performed in Coot34 , and refinement was performed in phenix.refine35 . All three chains are highly similar in structure, with all-atom pairwise RMSD of cytosolic domains ranging from 0.22 Å to 0.26 Å, while the orientation of the transmembrane helix relative to the soluble domain varies among protomers.
Assignment of sequence register was straightforward and unambiguous due to the relatively high resolution, almost completely ordered structure, and high frequency bulky amino acid side chains (σ1 receptor is roughly 5% tryptophan). As a control for register assignment, the structure was built and register assigned in two independent ways. First it was manually built and register assigned by inspection of electron density. In parallel, sequence register was independently assigned automatically with phenix.autobuild, and results were confirmed to be identical throughout the entire polypeptide chain of each protomer. Representative composite omit map density is shown inExtended Data Figure 2 . Ligands were manually placed into Fo−Fc difference maps (Extended Data Figure 7 ). In the case of PD144418 the electron density was clear, and ligand position and pose were unambiguous. For 4-IBP, the pose was unambiguous due to the high Fo−Fc peak resulting from the ligand iodine atom. Following refinement, structure quality was assessed using MolProbity36 , and figures were prepared in PyMOL37 and UCSF Chimera38 (link). All crystallographic data processing, refinement, and analysis software was compiled and supported by the SBGrid Consortium39 .
Initial phases were obtained in SHARP33 using single isomorphous replacement and anomalous scattering (SIRAS). Three transmembrane α-helices were identifiable in the initial map, suggesting three molecules in the asymmetric unit with an unusual solvent content of ~70%. Experimental phases were iteratively combined with model-derived phase to improve the electron density map through solvent flattening in SHARP. Model building was performed in Coot34 , and refinement was performed in phenix.refine35 . All three chains are highly similar in structure, with all-atom pairwise RMSD of cytosolic domains ranging from 0.22 Å to 0.26 Å, while the orientation of the transmembrane helix relative to the soluble domain varies among protomers.
Assignment of sequence register was straightforward and unambiguous due to the relatively high resolution, almost completely ordered structure, and high frequency bulky amino acid side chains (σ1 receptor is roughly 5% tryptophan). As a control for register assignment, the structure was built and register assigned in two independent ways. First it was manually built and register assigned by inspection of electron density. In parallel, sequence register was independently assigned automatically with phenix.autobuild, and results were confirmed to be identical throughout the entire polypeptide chain of each protomer. Representative composite omit map density is shown in