Fig. 2: A triad of histidine residues forms a zinc ion binding site on a Pra1 subunit on the exterior of the hexameric ring.

a The N-terminal and C-terminal ends of the protein are on opposite sides of Pra1. A single subunit consists of several short helices, as well as a long helix (24 amino acids in length), and a small beta-sheet. In addition, a Pra1 subunit harbors three disulfide bridges and four N-linked glycosylation sites. b A close-up of the histidine triad comprised of His 178, His 182, and His 193 in the presence of 1 mM ZnSO₄ reveals cryo-EM map density consistent with a metal ion coordinated by the triad. In this subunit (B), the distance from the center of the zinc ion to the center of the N3-nitrogen in the imidazole ring of the histidine residue is 2.3 Å for all three histidine sidechains. c The locations of the histidine triad consisting of His 178, His 182, and His 193 on the exterior ring are indicated in magenta space fill. Each subunit harbors one histidine triad, leading to six zinc ion binding sites on the exterior of the Pra1 ring. d A close-up of the histidine triad in MES pH 6.0 reveals a lack of metal ion binding under these conditions, as indicated by the absence of continuous cryo-EM density between the three histidines, in contrast to panel (b).