Journal
LANGMUIR
Volume 35, Issue 36, Pages 11735-11744Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.langmuir.9b01625
Keywords
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Funding
- Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy
- DOE scientific user facilities
- University of Windsor
- Ontario Graduate Scholarships (OGS)
- Natural Sciences and Engineering Research Council of Canada (NSERC) [RGPIN-2018-04841]
- NIH [R0IGM120642]
- NSF [MCB-1817929]
- Scientific User Facilities Division of the Department of Energy (DOE) Office of Science - Basic Energy Science (BES) Program, DOE Office of Science [DEAC05-00OR22725]
- U.S. Department of Energy [DE-AC05-00OR22725]
- Department of Energy
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Despite the prevalence of lipid transbilayer asymmetry in natural plasma membranes, most biomimetic model membranes studied are symmetric. Recent advances have helped to overcome the difficulties in preparing asymmetric liposomes in vitro, allowing for the examination of a larger set of relevant biophysical questions. Here, we investigate the stability of asymmetric bilayers by measuring lipid flip-flop with time-resolved small-angle neutron scattering (SANS). Asymmetric large unilamellar vesicles with inner bilayer leaflets containing predominantly 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and outer leaflets composed mainly of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) displayed slow spontaneous flip-flop at 37 degrees C (half-time, t(1/2) = 140 h). However, inclusion of peptides, namely, gramicidin, alamethicin, melittin, or pHLIP (i.e., pH-low insertion peptide), accelerated lipid flip-flop. For three of these peptides (i.e., pHLIP, alamethicin, and melittin), each of which was added externally to preformed asymmetric vesicles, we observed a completely scrambled bilayer in less than 2 h. Gramicidin, on the other hand, was preincorporated during the formation of the asymmetric liposomes and showed a time resolvable 8-fold increase in the rate of lipid asymmetry loss. These results point to a membrane surface-related (e.g., adsorption/insertion) event as the primary driver of lipid scrambling in the asymmetric model membranes of this study. We discuss the implications of membrane peptide binding, conformation, and insertion on lipid asymmetry.
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