Extended data Figure 5. JA2 and JB1 form homodimers and interact transiently.

a, Identified JA2 and JB1 inter-molecular cross-links; `Id', amino acid sequence of peptides showing cross-linked lysines (K, orange). Protein 1 and 2 denote source proteins for cross-linked peptides; position 1 and 2 denote positions of cross-linked lysines within proteins; deltaS is the delta score for each crosslink; cut-off = 0.9. ld-Score is the linear discriminant score. b, Representative mass spectrometry spectra for inter-molecular JA2 and JB1 cross-links. Common peaks, green; cross-linked, red; matched peaks, diamonds (no peaks above 1,100 m/z detected). c, SEC profiles of ³H-labelled JA2 dimer (green cartoon) and ³H-labelled JB1 dimer (blue cartoon) mixed with unlabelled J-protein from the other class. Precise concentrations are shown in Extended Data Table 1.