text: Crystal structures of the resting or ground states of AR1 (3.4 Å resolution), AR2 (1.8 Å resolution) and AR3 (1.07 and 1.3 Å) have been deposited in the Protein Data Bank. Proteins possess seven transmembrane α-helices and a two-stranded extracellular-facing β-sheet. Retinal is covalently bonded via Schiff base to a lysine residue on helix G. The conserved DLLxDGR sequence, close to the extracellular-facing N-terminus of both proteins, forms a tightly curved omega loop that has been implicated in bacterioruberin binding. The cleavage of the first 6 amino acids and the conversion of Gln7 to a pyroglutamate (PCA) residue was also observed in AR3, as previously reported for bacteriorhodopsin.