The structure of human MRP8, a member of the S100 calcium-binding protein family, by MAD phasing at 1.9 Å resolution

The structure of human MRP8 in the calcium-bound form was determined at 1.9 Angstrom resolution by X-ray crystallography. The structure was initially solved by MAD phasing of an ytterbium-substituted crystal and was refined against data obtained from a Ca2+-bound crystal. The dimeric form of MRP8 was stabilized by hydrophobic interactions between mutually wrapped helices. There were two EF-hand motifs per monomer and each EF-hand bound one Ca2+ with a different affinity [the affinity of the C-terminal EF-hand (EF-2) for Ca2+ was stronger than that of the N-terminal EF-hand (EF-1)]. Furthermore, replacement with Yb3+ occurred in the C-terminal EF-hand only, suggesting a more flexible nature for EF-2 than for EF-1. This, combined with previous observations that the helix in EF-2 (helix III) undergoes a large conformational change upon calcium binding, suggests that the C-terminal EF-hand (EF-2) plays a role as a trigger for Ca2+-induced conformational change.