Leaflet Structure and Bending Fluctuations

(funded by ILL graduate school and University of Graz)

start of project: October 1, 2018
end of project: Juni 30, 2022

Lipid asymmetry is a hallmark of biological membranes. In particular, prototypical mammalian plasma membranes are known to be composed of an outer leaflet enriched in cholinephospholipids, while the majority of the aminophospholipids are confined to the inner leaflet. One of the enduring questions concerning plasma membrane architecture and lipid asymmetry is the possibility of bilayer leaflets being coupled to each other. This coupling may influence a number of physiological processes that require communication between, for example, receptors secreted to the exoplasm and components of signal transduction pathways in the cytoplasm. Currently conceived lipid-mediated coupling mechanisms consider either intrinsic lipid curvature, headgroup electrostatics, cholesterol flip-flop, dynamic chain interdigitation [5,6], or thermal membrane fluctuations. Early experimental insight on asymmetric membranes was obtained from planar bilayers, and showed that depending on lipid composition interleaflet coupling may induce domains in the otherwise homogenous apposing leaflet. Asymmetric lipid vesicles, based on cyclodextrin (CD)-mediated lipid exchange have recently emerged as a viable and highly-useful platform to study transbilayer coupling mechanisms. A variety of asymmetric lipid vesicles showed independent melting of inner and outer leaflets, but some weak ordering of a fluid inner monolayer in the presence of a coexisting outer gel leaflet.

We propose a comprehensive study of asymmetric large unilamellar lipid vesicles (aLUVs) with defined chemical composition to determine whether or not a given leaflet senses the structure and collective fluctuations of its apposing leaflet. We expect fundamental insight into the biophysics pertaining to transbilayer leaflet coupling mechanisms in asymmetric membranes. The goals will be achieved by coupling elastic (small-angle neutron and X-ray scattering; SANS/SAXS) and inelastic neutron scattering (neutron spin-echo; NSE) techniques in combination with contrast variation. The specific focus will be on effects of hydrocarbon chain and headgroup composition, respectively.