Open AccessLight-independent phospholipid scramblase activity of bacteriorhodopsin from Halobacterium salinarum
Author(s) -
Alice Verchère,
Wei-Lin Ou,
Birgit Ploier,
Takefumi Morizumi,
Michael A. Goren,
Peter Bütikofer,
Oliver P. Ernst,
George Khelashvili,
Anant K. Me
Publication year - 2017
Publication title -
scientific reports
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.24
H-Index - 213
ISSN - 2045-2322
DOI - 10.1038/s41598-017-09835-5
Subject(s) - halobacterium salinarum , bacteriorhodopsin , phospholipid scramblase , transmembrane protein , trimer , transmembrane domain , chemistry , biochemistry , phospholipid , flippase , vesicle , biophysics , lipid bilayer , biology , membrane , phosphatidylserine , dimer , receptor , organic chemistry
The retinylidene protein bacteriorhodopsin (BR) is a heptahelical light-dependent proton pump found in the purple membrane of the archaeon Halobacterium salinarum . We now show that when reconstituted into large unilamellar vesicles, purified BR trimers exhibit light-independent lipid scramblase activity, thereby facilitating transbilayer exchange of phospholipids between the leaflets of the vesicle membrane at a rate >10,000 per trimer per second. This activity is comparable to that of recently described scramblases including bovine rhodopsin and fungal TMEM16 proteins. Specificity tests reveal that BR scrambles fluorescent analogues of common phospholipids but does not transport a glycosylated diphosphate isoprenoid lipid. In silico analyses suggest that membrane-exposed polar residues in transmembrane helices 1 and 2 of BR may provide the molecular basis for lipid translocation by coordinating the polar head-groups of transiting phospholipids. Consistent with this possibility, extensive coarse-grained molecular dynamics simulations of a BR trimer in an explicit phospholipid membrane revealed water penetration along transmembrane helix 1 with the cooperation of a polar residue (Y147 in transmembrane helix 5) in the adjacent protomer. These results suggest that the lipid translocation pathway may lie at or near the interface of the protomers of a BR trimer.