Overlay of the four essential residues (Gln, Arg, Glu, Tyr) of S. pombe GIIβ (orange, PDB ID: 2LVX), human OS-9 (gray, PDB ID: 3AIH), bovine CD-MPR (magenta, PDB ID: 1C39), bovine CI-MPR MRH domain 3 (green, PDB ID: 1SZO), bovine CI-MPR MRH domain 5 (blue, PDB ID: 2KVB). Disulfide bridges are shown in yellow and loops C and D are labeled.