text: Bacteriorhodopsin is a 27 kDa integral membrane protein usually found in two-dimensional crystalline patches known as "purple membrane", which can occupy almost 50% of the surface area of the archaeal cell. The repeating element of the hexagonal lattice is composed of three identical protein chains, each rotated by 120 degrees relative to the others. Each monomer has seven transmembrane alpha helices and an extracellular-facing, two-stranded beta sheet.
  Bacteriorhodopsin is synthesized as a protein precursor, known as bacterio-opsin, which is extensively modified after translation. The modifications are:
  * Covalent conjugation of a retinal molecule to residue Lys216, via a Schiff base, to create the retinylidene chromophore.
  * Cleavage of the signal peptide, the first 13 amino acids at the N-terminus, and the conversion of residue Gln14 to pyroglutamate
  * Removal of residue Asp262 at the C-terminus