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, a Ca2+/Mg2+ exchange may rarely occurs in vivo due to relatively low concentration and the balance of these two ions in the extracellular fluids. These observations confer to the presence of Ca2+ in the central pore an important, albeit indirect, role in the complex formation. Some conformational motion of the myocilin-OLF equivalent loop B-10/C-11 has been reported when comparing the polyethylene glycol-containing wild type structure to the glycerol-containing form or to the SeMet derivative form of the E396D mutant (Donegan et al., 2014). Gliomedin-OLF appears to be different in this respect, since the LPHN3-OLF Tyr323-Asp332-Ca2+ trio structurally superimposes with Phe415-Asn423-Na+ (Figure S1D, related to figure 2), however, the gliomedin-OLF 407–420 loop (equivalent to LPHN3-OLF 316–329) is poorly conserved (Han and Kursula, 2015). The fact that Phe415 and the proximal Leu414 are involved in a core-buried hydrophobic cluster suggests that the entire loop is less dynamic. In general, solvent accessibility of the OLF β-propeller central cavity appears to be a shared
