Thermotropic Phase Transition in Soluble Nanoscale Lipid Bilayers

Abstract

The role of lipid domain size and protein-lipid interfaces in the thermotropic phase transition of dipalmitoyl phosphatidylcholine (DPPC) and dimyristoyl phosphatidylcholine (DMPC) bilayers in Nanodiscs was studied using small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and generalized polarization (GP) of the lipophilic probe Laurdan. Nanodiscs are watersoluble, monodisperse self-assembled lipid bilayers encompassed by a helical membrane scaffold protein (MSP). MSPs of different lengths were used to define the diameter of the Nanodisc lipid bilayer from 76 to 108 Å and the number of DPPC molecules from 164 to 335 per discoidal structure. In Nanodiscs of all sizes, the phase transitions were broader and shifted to higher temperatures relative to those observed in vesicle preparations. The size dependences of the transition enthalpies and structural parameters of Nanodiscs reveal the presence of a boundary lipid layer in contact with the scaffold protein encircling the perimeter of the disc. The thickness of this annular layer was estimated to be approximately 15 Å, or two lipid molecules. SAXS was used to measure the lateral thermal expansion of Nanodiscs and a steep decrease of bilayer thickness during the main lipid phase transition was observed. These results provide basis for the quantitative understanding of cooperative phase transitions in membrane bilayers in confined geometries at the nanoscale.

Cholesterol is a determinant of the structures of discoidal high density lipoproteins formed by the solubilization of phospholipid membranes by apolipoprotein A-I

“…At 37 °C, DPH polarization values where higher in rHDL than in MLV however the change in polarization with mol% cholesterol was identical for rHDL and MLV ( Figure 8C). These polarization results are consistent with previous models that indicate two different bilayer environments for DMPC [44][45][46]. These are a boundary phospholipid bilayer (estimated to be the width of two phospholipid molecules) [46] that excludes cholesterol and whose dynamic motions are perturbed because they are adjacent to the protein [45] and a bulk lipid fraction that has similar motional properties of DMPC/ cholesterol in MLV.…”

Transducin Activation by Nanoscale Lipid Bilayers Containing One and Two Rhodopsins

“…Importantly, excess amphiphile, either detergent or lipid, is detrimental to G-coupling assays, and thus care was taken to remove these and bare Nanodiscs from the samples. The largest well characterized Nanodisc formed with the membrane scaffold protein MSP1E3D1 was (4) 22 (3) 3 (1) 10 ( used to provide sufficient bilayer area to incorporate putative dimers (16,17,37). A chemically defined and pure phospholipid and affinity-purified rhodopsin were used to form the Nanodiscs, from which we have isolated separate populations of Nanodiscs containing one and two rhodopsin molecules.…”

Coarse Grained Protein−Lipid Model with Application to Lipoprotein Particles

“…The diameters of the nanodiscs, 9.5 nm at 300 K, 10.5 nm at 323 K, and 10.6 nm at 353 K (as measured from the CG simulations) did, in fact, increase with temperature and correlated well with experimental SAXS measurements (see Table 4). 25,26 The gaps between the ends of membrane scaffold protein strands increase with temperature suggesting that perhaps disassembly of nanodiscs results from the loss of favorable salt-bridging (represented in our CG model through CG particle charges) between the two scaffold protein strands (Figure 5d, e, and f).…”

“…Membrane scaffold proteins are engineered based on the structure of the lipid binding domain of apo A-1. Nanodiscs have been well characterized 25,26,27 and in addition to acting as platforms for studying membrane proteins, their well-defined size and composition provide an ideal system for studying the HDL lipid-binding domain.…”