The model refinement of the P65 crystal form indicated that the cation Ca2+ binding site was not fully occupied by the Ca2+, which was refined with 70% occupancy. Although partial exchange of Ca2+ by Mg2+ could have occurred in the P65 form, our crystallographic data are not sufficient to definitively conclude what proportion of Ca2+/Mg2+ composes the mixture. However, as the physiological concentration of Mg2+ in the extracellular fluids is 0.7–1mM (Bringhurst et al., 2012), the presence of Mg2+ in the C2221 crystal structure is most likely a crystallization artifact. Recently determined structures of myocilin-OLF (Donegan et al., 2014) and LPHN3-RBL/OLF (Jackson et al., 2015), contain the same Ca2+ binding site and a Na+ in the central pore that we could not identify from the electron density maps probably due to a partial occupancy (like Ca2+) as indicated by the alternative conformation of Asp436 (P65) (Figure 2E, Figure S1B, related to figure 2). Hence, a joint analysis of the two crystal forms of LPHN3-OLF and the LPHN3-RBL/OLF structure reveals that Ca2+ and Na+ both contribute to the conformational stability of
