Consequently to the opening of the 316–329 loop (C2221), the β-propeller central cleft becomes solvent-accessible from the “entry” face (Figure 2D), with a total volume of accessible area of 1550 Å3, as opposed to the value of the closed state (P65) of only 220 Å3 of a buried cavity, disconnected from the surface of the protein. By examining the geometric characteristics of the coordination sphere of a heteroatom found in the central cleft, and exploiting the anomalous signal from the dataset collected at low energy through an f″ refinement (Liu et al., 2013), we identified a Ca2+in the closed state (Figure 2E). The shorter cation-oxygen distances within the octahedric coordination geometry of the open state (Figure 2E) clearly designate a Mg2+ ion (Harding, 2006) also validated by CheckMyMetal (Zheng et al., 2014). Hence, with the presence of 300 mM of Mg2+ used in crystallization, Ca2+ was replaced by Mg2+. In the closed state, the Ca2+ bridges three central cleft β-strands through the involvement of residue atoms in its coordination: Asp332 Oδ2 (β10), Asn380 Oδ1 (β14) and Val435 O (β18). Additionally,
