Evaluation of diacylphospholipids as boundary lipids for bacteriorhodopsin from structural and functional aspects

https://doi.org/10.1016/j.bbamem.2016.06.006Get rights and content
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Highlights

  • Bacteriorhodopsin (bR) was reconstituted in phospholipid membrane.

  • 31P NMR showed the stronger interaction between bR and phosphatidylglycerol (PG).

  • PG and phosphatidic acid (PA) better restored the bR photocycle.

  • Unsaturation in acyl chains of PG affected the photocycle restoration.

Abstract

Reconstituted membranes with diverse diacylphospholipids were prepared by using bacteriorhodopsin (bR) in which the intrinsic lipid content was decreased to 24% of the original while the trimeric structure and photocycle of bR were retained. Four phospholipids with a different headgroup, phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylserine (PS), were adopted for reconstitution. By varying the lipid-protein ratios, the interactions of these phospholipids with bR, as a boundary lipid, were evaluated by solid state 2H/31P NMR, circular dichroism (CD), and laser-flash photolysis. The 31P NMR results revealed that the headgroup of acidic phosphatidylglycerol (PG) interacts more strongly with bR than that of phosphatidylcholine (PC). CD analysis indicated that the trimetric structure of bR was retained in all the phospholipid-bR preparations at low and medium lipid contents. Acidic lipids PA, PG and PS restored the photocycle activity of bR to an extent comparable to (or slightly lower than) that of the purple membrane while PC caused a marked reduction of the bR photocycle efficiency. Among PGs with different fatty acyl groups, those with mono- and di-unsaturated lipids tended to preserve the photocycle efficiency, whereas the fully saturated lipid did not. These results show that acidic unsaturated phospholipids, particularly dioleoylphosphatidylglycerol (DOPG), have higher affinity for bR and efficiently restore its trimetric structure. The present study suggests that bR reconstituted in DOPG bilayers may possibly be used as a model system for spectroscopic investigations of the lipid-bR interactions with the membrane-integral α-helices, and potentially for a similar type of membrane proteins.

Abbreviations

bR
bacteriorhodopsin
dbR
delipidated bacteriorhodopsin
DM
n-decyl-β-d-maltopyranoside
DOPA
1,2-dioleoylphosphatidic acid
DOPC
1,2-dioleoylphosphatidylcholine
DOPG
1,2-dioleoylphosphatidylglycerol
DOPS
1,2-dioleoylphosphatidylserine
DSPG
1,2-distearoylphosphatidylglycerol
GlyC
archaeal glycocardiolipin or 3-HSO3-Galp-β1,6-Manp-α1,2-Glcp-α1,1-[sn-2,3-di-O-phytanylglycerol]-6-[phospho-sn-2,3-di-O-phytanylglycerol]
GPCR
G-protein coupled receptor
LFP
laser-flash photolysis
PC
phosphatidylcholine
PG
phosphatidylglycerol
PGP-Me
phosphatidylglycerophosphate methyl ester
PGS
archaeal phosphatidylglycerosulfate
PM
purple membrane
POPG
1-palmitoyl-2-oleoyl-glycerophosphoglycerol
STeGA
3-HSO3-Galp-β1,6-Manp-α1,2(Galf-α1,3)-Glcp-α1,1-sn-2,3-diphytanylglycerol
S-TGA-1
3-HSO3-Galp-β1,6-Manp-α1,2-Glcp-α1,1-sn-2,3-diphytanylglycerol
(18:2)2PG
dilinoleoylglycerophosphoglycerol

Keywords

Boundary lipid
Lipid-protein interaction
Bacteriorhodopsin
Solid state NMR
Phosphatidylglycerol
Unsaturated acyl groups

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The authors declare that they have no conflicts of interest.