FIG 1.
Features of ActRs from acI clades A, B, and C. (A) ActRs from clade A (ActRO22), clade B (ActRL06), and clade C (ActRMEE578) are aligned to a homolog, xantho-opsin (XR) from Salinibacter ruber. Proteins are subscripted with genome shorthand (Table S1). Features are highlighted: predicted helices (gray, numbered), cysteines (yellow), main proton shuttles (orange), main absorbance tuner (green), Schiff base lysine (magenta), possible antenna carotenoid residues (cyan), and proline indicative of ActR versus xantho-opsins (red). Antenna carotenoid residues are based on amino acids in proximity to salinixanthin in the crystal structure of xanthorhodopsin (PDB 3DDL). (B) Three-dimensional (3D) structure prediction for ActR with key residue side chains displayed according to the color coding in panel A. The α-carbon of glycine near the top of α5 that would allow antenna binding is shown as a sphere, the Schiff base retinal (lime green) and predicted disulfide bond in clades A and B are modeled, and the N and C termini are labeled.