Comparison of bacteriorhodopsin/phospholipid interactions in DMPC and DMPG bilayers: an electron spin resonance...

Bacteriorhodopsin (BR), separated from the Halobacterium halobium purple membrane (PM) lipids, broadened the main gel-fluid phase transition (Tc) of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers and 1,2-dimyristoyl-sn-glycero-3-phosphoryl-3'-rac-glycerol (DMPG) bilayers to similar degrees compared to the respective protein-free bilayers. Analysis of two component ESR spectra from spin-labelled phospholipids suggested that, at temperatures above Tc, each 26 kDa BR was associated with 18-21 boundary lipids in both DMPC and DMPG bilayers. This was consistent with the random dispersive properties of the BR particles observed by freeze-fracture electron microscopy in DMPG and DMPC bilayer complexes when quenched from temperatures above Tc. DMPG, as shown for DMPC in other studies, did not aggregate BR into the 2D-hexagonal arrays of PM at temperatures below Tc. Despite the presence of positively-charged amino acid residues which project from the BR into the polar headgroup region of the phospholipid bilayer, BR exhibited a lower affinity for the negatively-charged phosphoglycerol headgroup compared to the zwitterionic phosphocholine headgroup, suggesting that ionic interactions between BR and phospholipid headgroups are not significant and serve a passive function, perhaps facilitating the close packing of BR into 2D-arrays.