In LPHN3-OLF, our crystallographic study designates loop 316–329 as one of the structural elements stabilized in presence of Ca2+ (the closed state), indirectly via a hydrogen bond between Asp332 and Tyr323. Moreover, this loop is involved in the binding interface with FLRT3-LRR as also shown by DXMS and in fact the open state of this loop is incompatible with the binding to FLRT3- LRR. However, loop 316–329 was found to be dynamic both in cristallo and in solution as shown by DXMS data of LPHN3-OLF alone (Figure 3A), which allows solvent exchange in the β-propeller central pore, evidenced by the C2221 crystal form. The high concentration of Mg2+ used for the crystallization of the C2221 crystal form led to a complete Ca2+/Mg2+ exchange in the central pore and to a stable enough conformation to be crystallized. Due to the well-established specific (de-)hydration properties of the Mg2+ ion as compared to Ca2+ (Maguire and Cowan, 2002), the ion coordination with Asp332 probably could not be established, maintaining loop 316–319 in an open state. Although loop 316–329 is dynamic under physiological condition,
