a, Lysine residues modified within the NTSR1–βarr1(ΔCT) complex after treatment with sulfo-LC-SDA were identified by mass spectrometry. Although mass spectrometry data were collected for each band observed by SDS–PAGE separately (Extended Data Fig. 3b), data were combined for analysis as the overall sample was imaged by cryo-EM. For each lysine residue in the complex the degree of modification was approximated as the percentage of times a lysine was found to be modified relative to the total number of times that lysine is observed across all peptides. The Cα for lysine residues that were observed to be modified less than 30% of the time are shown as grey spheres, whereas those found to be modified more than 30% of the time are shown as green spheres. A tabulated analysis is presented in Supplementary Tables 1–7. b, Of the modified lysine residues, several mainly formed dead-end crosslinks (reaction of the activated diazirene with water); those with more than 30% of the lysine residues resulting in dead-end crosslinks are shown as maroon spheres (Cα). c, Only one intermolecular crosslink is observed that is consistent with the structural model obtained from cryo-EM. This crosslink was localized to ICL1 (K91) and a β-strand (shown in green) in the arrestin C-lobe. d, Several intramolecular crosslinks within ICL2 and the adjacent helices are observed. Reactive lysine residues are shown as green spheres (for Cα), and crosslinks were localized to the region shown in green. e, Multiple intramolecular crosslinks within arrestin were observed. Each reactive lysine residue (shown as a Cα sphere) that forms an intrapeptide crosslink, or an intramolecular (within arrestin) crosslink, is coloured to match the region in which the observed crosslink could be localized. Each band was analysed once by mass spectrometry from a representative cryo-EM preparation.