Comparison of Bacteriorhodopsin/Phospholipid Interactions in DMPC and DMPG Bilayers: An Electron Spin Resonance Spectroscopy and Freeze-Fracture Electron Microscopy Study

Abstract

Bacteriorhodopsin (BR), separated from the Halobacterium halobium purple membrane (PM) lipids, broadened the main gel-fluid phase transition ( T c ) 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 T c , 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 T c . DMPG, as shown for DMPC in other studies, did not aggregate BR into the 2D-hexagonal arrays of PM at temperatures below T c . 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.