Deuteration levels of LPHN3-OLF in complex with FLRT3-LRR should reveal whether some of the loops belonging to the binding interface become more solvent protected. In fact, LPHN3-OLF loop 316–329 in complex with FLRT3-LRR is significantly less deuterated than in the free protein (Figure 3C,D). These data suggest that loop 316–329 is stabilized in a solvent-inaccessible area of the complex (see below). To a lesser extent, fragment 396–400 also shows a high level of deuteration indicating a large degree of flexibility (>50% deuteration is 100s) consistent with the absence of this loop in the crystallographic models (Figure S2A,B, related to figure 3; Table S1, related to Figure 2). Other neighboring and superficial fragments show various degree of solvent protection (Phe217-Trp226; Met240-Thr247; Asn289-Val297; Arg376-Met383; Tyr429-Ala433; Glu401-Tyr409) whereas loops that are distant from the putative binding surface show higher deuteration exposure (Lys228-Ala233; Thr303-Ala310) (Figure 3E), in agreement with the interfacing area revealed by crystallography.
