Comparison of bacteriorhodopsin/phospholipid interactions in DMPC and DMPG bilayers: an electron spin resonance spectroscopy and freeze-fracture electron microscopy study.
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.
Gale
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Department of Biochemistry, University of Oxford, U.K.
D001436:Bacteriorhodopsins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D005614:Freeze Fracturing; D019617:Halobacterium salinarum / Q000378:metabolism*; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism*; D008854:Microscopy, Electron / Q000379:methods; D010715:Phosphatidylglycerols / Q000737:chemistry / Q000378:metabolism*; D013816:Thermodynamics
D008051:Lipid Bilayers; D010715:Phosphatidylglycerols; D001436:Bacteriorhodopsins; C002773:dimyristoylphosphatidylglycerol; D004134:Dimyristoylphosphatidylcholine