Chemical and functional studies on the importance of purple membrane lipids in bacteriorhodopsin photocycle behavior

In native purple membrane (PM), there are approximately 1 squalene, 2 glycolipid sulfate (GLS), and 6 phospholipid (PL) molecules per bacteriorhodopsin (BR) monomer [10]. Brief (∼ 2 min) exposure to 0.1% Triton X-100 removes about 25%, 20%, and 6% of squalenes, GLS, and PL, respectively (this paper) while causing profound changes in the BR photocycle, including the loss of ‘photocooperativity’ [1]. The BR photocycle in Triton-treated PM can be restored to near normal behavior by reconstitution with native PM lipids. Isolated squalenes are not effective whereas PL alone partially restores normal photocycle characteristics.