Bacteriorhodopsin

Bacteriorhodopsin is isolated from Halobacterium salinarum, a purple-colored member of the Archaea kingdom, which grows in salty water.

Bacteriorhodopsin is trimeric. One monomer is colored blue.

Essen L, Siegert R, Lehmann WD, Oesterhelt D.
Lipid patches in membrane protein oligomers: crystal structure of the bacteriorhodopsin-lipid complex.
Proc Natl Acad Sci U S A. 1998 Sep 29;95(20):11673-8. PubMed

1br (PDB)