Boundary lipid in proteolipid-lipid recombinants revealed by fluorescence energy transfer and spin probe study

Composition, structure and substrate properties of reconstituted discoidal HDL with apolipoprotein A-I and cholesteryl ester

To investigate the influence of lipid unsaturation and neutral lipid on the maturation of high density lipoproteins, the discoidal complexes of apoA-I, phosphatidylcholine and cholesteryl ester (CE) were prepared. Saturated dipalmitoylphosphatidylcholine (DPPC) and unsaturated palmitoyllinoleoylphosphatidylcholine (PLPC), palmitoyloleoylphosphatidylcholine (POPC), and fluorescent probe cholesteryl 1-pyrenedecanoate (CPD) that forms in a diffusion- and concentration-dependent manner short-lived dimer of unexcited and excited molecules (excimer) were used. The apoA-I/DPPC/CPD complexes were heterogeneous by size, composition and probe location. CPD molecules incorporated more efficiently into larger complexes and accumulated in a central part of the discs. The apoA-I/POPC(PLPC)/CPD were also heterogeneous, however, probe molecules distributed preferentially into smaller complexes and accumulated at disc periphery. The kinetics of CPD transfer by recombinant cholesteryl ester transfer protein (CETP) to human plasma LDL is well described by two-exponential decay, the fast component with a shorter transfer time being more populated in PLPC compared to DPPC complexes. The presence of CE molecules in discoidal HDL results in particle heterogeneity. ApoA-I influences the CETP activity modulating the properties of apolipoprotein-phospholipid interface. This may include CE molecules accumulation in the boundary lipid in unsaturated phosphatidylcholine and cluster formation in the bulk bilayer in saturated phosphatidylcholine.

Thermotropic phase transition in soluble nanoscale lipid bilayers

The role of lipid domain size and protein-lipid interfaces in the thermotropic phase transition of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (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 water-soluble, 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 A 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 A, 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 the basis for the quantitative understanding of cooperative phase transitions in membrane bilayers in confined geometries at the nanoscale.

Evaluation of age-related changes of physicochemical properties and functional activity of rat adipose plasma membranes and their possible relationship

Studies were carried out to evaluate structural state of adipose plasma membranes (PM) from mature adult and aged Wistar rats and its possible relation to functional changes in aging. PM lipids were probed by fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH) and pyrene. The data on DPH anisotropy, pyrene excimerization and induction resonance energy transfer (IRET) from PM proteins to pyrene provided the evidence for age-dependent decrease in PM lipid phase fluidity inclusive of that of annular lipid. Interaction between PM lipids and integral proteins also changed in aging. The observed structural changes were due to age-related lipid compositional modification. Evidence for this conclusion was provided by the following data: (i) DPH anisotropy was increased in aging both in PM and liposomes from the same PM lipids; (ii) both saturation/unsaturation fatty acid ratio and relative content of phosphatidylethanolamine (PE) increased in aged PM, whereas cholesterol/phospholipid and lipid/protein ratios were age-independent; (iii) PM polypeptide composition remained practically unchanged during aging. Age-related changes of the PM structure were accompanied by a twofold decrease of high affinity insulin binding sites and elevation of their affinity to insulin. It was suggested that age-related changes of physicochemical properties of PM lipid phase might affect protein molecular conformation and function through either changes in bulk lipid fluidity or through specific lipid-protein interactions.

Non-uniform distribution of phospholipids in (Na+ + K+)-ATPase-rich membranes from Torpedo marmorata electric organ evidenced by spin-spin interactions between spin-labeled phospholipids

(Na+ + K+)-ATPase membranes from Torpedo marmorata electric organ were labeled with high concentrations of paramagnetic phospholipid analogs (up to 5 mol of spin labels for 100 mol of endogeneous phospholipids). The mobile lipid bilayer component of the complex resonance spectra obtained showed low-field linewidth broadening due to spin-spin interactions; this was used as a measure of probe concentration in this compartment. Our results show that in the lipid shell surrounding the protein, there is a considerable enrichment in negatively charged phospholipids over neutral ones.