To obtain dynamic information on the individual structures and their complex, we used DXMS. After optimization of the mass spectrometry (MS) coverage of the individual proteins (data not shown), high resolution MS data we collected on LPHN3-OLF alone and bound to FLRT3-LRR. Deuteration levels of LPHN3-OLF alone show that the first ~20 amino acids (APSTDHLDYKDDDDKAAAKV) and the last ~40 amino acids (LDSRSGPVHHGQVSYISPPIHLDSELERPPVRGILEVLFQ) of the protein exchanged >90% deuteron in the first 10 seconds, indicating that these regions are fully solvent exposed and disordered. These 60 residues are clipped during crystallization trials and could not be built in our structure. Interestingly, among the LPHN3-OLF fragments that acquire deuteration over time (see supplemental information, Figure S2 and S3, related to figure 3), loop 316–329 is gradually deuterated and acquires >50% deuteration in 100s, reaching ~ 90 % deuteration after 10,000s (Figure 3A). This increase in deuteration level indicates a progressive solvent exposure of the amide hydrogens. In light of the crystallographic models, this strongly suggests that loop 316–329 is dynamic, as a result of a conformational motion between the closed and open
