Fig. 3: Structural similarities between PufX and the N-termini of the RC-bound Cyt subunits.

a Superposition of the Cα carbons of the RC-M subunits between the Rba. sphaeroides monomeric LH1–RC (PDB: 7F0L) and Rpi. globiformis RC (this work). PufX is shown as orange cartoon. The transmembrane helix and the following loop region of the Cyt subunit are shown by cyan cartoon and green sticks (main chain only), respectively. The other portion of the Cyt subunit is shown by a black ribbon and heme groups are shown by pale yellow sticks. Protein-U in Rba. sphaeroides LH1–RC is shown by magenta ribbon. b Expanded view marked in a showing that the C-terminus (Leu55–Gly69) of PufX overlaps positionally and conformationally with the N-terminal loop region (Phe27–Arg40, colored green) of the Rpi. globiformis Cyt subunit. c Similar comparison between the C-terminus of Rba. sphaeroides PufX and the N-terminus (Cys21–Phe34, colored green) of Blc. viridis Cyt subunit (PDB: 3T6E). d Similar comparison between the C-terminus of Rba. sphaeroides PufX and the N-terminus (Cys23–Tyr35, colored green) of Tch. tepidum Cyt subunit (PDB: 5Y5S). e Comparison of the Rba. sphaeroides PufX with the N-terminal transmembrane domain (Arg17–Ala49, red cartoon) of Rof. castenholzii Cyt subunit in an LH–RC structure (PDB: 5YQ7). A hypothetical subunit X is shown by magenta cartoon. f Sequence alignments of Rba. sphaeroides PufX (from strain IL106) with the N-termini of the Cyt subunits from other phototrophs based on the residues in the loop regions marked by dashed box or the transmembrane domains. Residues in the C-terminus (Leu55–Gly69) of PufX are shown by orange fonts. Residues truncated by posttranslational modification are shown by gray fonts. The Cys residues that bind a triacyl or diacyl group are shown by red fonts. Residues with green fonts correspond to the N-terminal portions shown in a–d. Transmembrane regions are underlined.