(A, B) Peripheral stalk architectures of S. cerevisiae (A) (Guo et al., 2017) and E. gracilis (B) viewed from the F1 (not shown) towards the peripheral stalk. The two structures have subunits b, d, 8 and OSCP in common. Unlike in S. cerevisiae, subunit f does not contribute to the peripheral stalk in E. gracilis. Subunit F6 is not found in E. gracilis. Red arrowheads indicate different heights of OSCP attachment to subunit b and subunit d respectively. (C) The C-terminal extension of E. gracilis OSCP (red rectangle) extends in between the peripheral stalk (dark grey) and F1 (light grey). (D) Close-up of OSCP/subunit d interaction in the E. gracilis peripheral stalk. The indicated residues are conserved in Euglenozoa. (E) The E. gracilis OSCP contains conserved structural elements (light green)(Srivastava et al., 2018) with the N-terminal domain consisting of helices 1,2,5 and 6 and the C-terminal domain consisting of a four-stranded β-sheet and helices 7 and 8 (dihydrolipicolinate reductase domain 2-like fold). The N-terminal extension (light yellow) interacts with subunit γ (blue) of the rotor, whereas the C-terminal extension (orange) is anchored to the rest of the peripheral stalk. (F-H) Multiple sequence alignments of the OSCP C-terminal region (F), with the Euglenozoa-specific extension highlighted red; OSCP N-terminal region (G) with the N-terminal helix found in the E. gracilis OSCP highlighted yellow; red rectangles indicate predicted cleavage sites of mitochondrial processing peptidase. (H) Subunit γ C-terminal region, with euglenozoa-specific extension highlighted red; Bos tarus (Bt), Homo sapiens (Hs), Caenorhabditis elegans (Ce), Saccharomyces cerevisiae (Sc), the trypanosomatids Trypanosoma brucei (Tb), Leishmania major (Lm), Endotrypanum monterogeii (Em), Crithidia fasciculata (Cf), Leptomonas pyrrhocoris (Lp), Blechomonas ayalai (Ba), Phytomonas sp. (Ps) and the euglenoid Euglena gracilis (Eg).