Fig. 2: Cryo-EM structure of CIII2CIV.
a The structures of CIII2 (PDB: 6XI0) and the homology model of R. capsulatus cbb3-type CIV were fitted into the cryo-EM map SC-2A (gray), depicted as a side view. All subunits are colored and labeled as in Fig. 1, and the arrow at the edge of CcoN of CIV is for the extra N-ter TMH (TMH0, light purple) specific to R. capsulatus. b Representative regions of the cryo-EM map showing map quality and model fitting. The TMH2 and TMH10 of CcoN show heme b and some bulky side chains. The protein backbone and hemes bL and bH are resolved between cyt b TMH2–TMH4 (compare to Fig. 3b, CIII2 map at 3.3 Å). Large side chains are visible between the TMH4s of CIII2 monomers A and B (4A and 4B). c Top view of CIII2CIV with TMHs depicted as cylinders and colored as in a. The TMHs of cyt b (only CIII2 monomer A) and CcoN of CIV are numbered, and the TMHs of the FeS protein (yellow), cyt c1 (green), CcoO (dark green), CcoP (light blue), CcoH/cy (blue/red with an arrow), and CcoN TMH0 (light purple) are shown. d 180° rotated view for the back view of CIII2CIV interface. The two extra TMHs at the interface are those of CcoH (blue) and cyt cy (red). e Enlarged view of CIII2CIV interface. The view is slightly rotated relative to d for better visibility of CcoP TMH in the background (light blue). CcoN TMH9 is next to CcoH and cyt cy TMHs. The fusion region between cyt c1 and CcoP is indicated at the bottom, with cyt c1 C-ter (green) and CcoP N-ter (light blue) with their respective end residues (Lys257c1 and Thr13CcoP) (only resolved portions). The 12 N-ter CcoP residues connecting these two chains (dashed line) are not resolved. f Enlarged view showing close interaction between CcoH and cyt cy TMHs. Characteristic features of cyt cy TMH (NH2-Gly11xxxPhe15xxxxxTyr21-COOH) are used to determine the registration, with the helix break induced by Gly11, and bulky side chains for Phe15 and Tyr21 clearly visible.
