Smaller Discoidal High-Density Lipoprotein Particles Form Saddle Surfaces, but Not Planar Bilayers

Miyazaki, Masakazu; Nakano, Minoru; Fukuda, Masahiro; Handa, Tetsurou

doi:10.1021/bi900785x

Cited by 31 publications

(51 citation statements)

References 46 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Through a combination of computation and experimentation, we provide evidence that apoA-I accomplishes this feat of unique flexibility by the incremental twisting or untwisting of a saddle-shaped apoA-I double belt structure that creates minimal surface patches of lipid bilayer. These results, especially the excellent fit of the simulations to the 1 H NMR data, combined with the recent confirmation in reconstituted particles by Miyazaki et al (40) of saddle-shaped minimal surfaces in reconstituted PLrich HDL particles, provide strong evidence for the global correctness of the saddle-shaped structures reported here and in our previous computer simulations (14,16).…”

Section: Discussionsupporting

confidence: 87%

Structures of Discoidal High Density Lipoproteins

Jones

Chen

et al. 2010

Journal of Biological Chemistry

View full text Add to dashboard Cite

Conversion of discoidal phospholipid (PL)-rich high density lipoprotein (HDL) to spheroidal cholesteryl ester-rich HDL is a central step in reverse cholesterol transport. A detailed understanding of this process and the atheroprotective role of apolipoprotein A-I (apoA-I) requires knowledge of the structure and dynamics of these various particles. This study, combining computation with experimentation, illuminates structural features of apoA-I allowing it to incorporate varying amounts of PL. Molecular dynamics simulated annealing of PL-rich HDL models containing unesterified cholesterol results in double belt structures with the same general saddle-shaped conformation of both our previous molecular dynamics simulations at 310 K and the x-ray structure of lipid-free apoA-I. Conversion from a discoidal to a saddle-shaped particle involves loss of helicity and formation of loops in opposing antiparallel parts of the double belt. During surface expansion caused by the temperature-jump step, the curved palmitoyloleoylphosphatidylcholine bilayer surfaces approach planarity. Relaxation back into saddle-shaped structures after cool down and equilibration further supports the saddle-shaped particle model. Our kinetic analyses of reconstituted particles demonstrate that PL-rich particles exist in discrete sizes corresponding to local energetic minima. Agreement of experimental and computational determinations of particle size/shape and apoA-I helicity provide additional support for the saddle-shaped particle model. Truncation experiments combined with simulations suggest that the N-terminal prolinerich domain of apoA-I influences the stability of PL-rich HDL particles. We propose that apoA-I incorporates increasing PL in the form of minimal surface bilayers through the incremental unwinding of an initially twisted saddle-shaped apoA-I double belt structure.

show abstract

Section: Discussionsupporting

confidence: 87%

Structures of Discoidal High Density Lipoproteins

Jones

Chen

et al. 2010

Journal of Biological Chemistry

View full text Add to dashboard Cite

show abstract

“…Relative to the 9.6-nm disc (70∕1 mol∕mol POPC/apoA-I), the 7.8-nm disc (28∕1 mol∕mol POPC/apoA-I) has fewer PL acyl chains that require protection from exposure to the aqueous phase by interaction with the amphipathic α-helices of apoA-I. There are some morphological differences between the two sizes of HDL particles (46,47), but to a first approximation both can be considered as discs so that the 7.8-nm particle has an approximately 20% smaller perimeter. It follows that the two apoA-I molecules accommodated around the edge of a 7.8-nm disc are relatively crowded, compared to the 9.6-nm disc.…”

Section: Discussionmentioning

confidence: 99%

Apolipoprotein A-I helical structure and stability in discoidal high-density lipoprotein (HDL) particles by hydrogen exchange and mass spectrometry

Chetty

Mayne

Kan

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

130

View full text Add to dashboard Cite

To understand high-density lipoprotein (HDL) structure at the molecular level, the location and stability of α-helical segments in human apolipoprotein (apo) A-I in large (9.6 nm) and small (7.8 nm) discoidal HDL particles were determined by hydrogen-deuterium exchange (HX) and mass spectrometry methods. The measured HX kinetics of some 100 apoA-I peptides specify, at close to amino acid resolution, the structural condition of segments throughout the protein sequence and changes in structure and stability that occur on incorporation into lipoprotein particles. When incorporated into the large HDL particle, the nonhelical regions in lipid-free apoA-I (residues 45-53, 66-69, 116-146, and 179-236) change conformation from random coil to α-helix so that nearly the entire apoA-I molecule adopts helical structure (except for the terminal residues 1-6 and 237-243). The amphipathic α-helices have relatively low stability, in the range 3-5 kcal∕mol, indicating high flexibility and dynamic unfolding and refolding in seconds or less. A segment encompassed by residues 125-158 exhibits bimodal HX labeling indicating co-existing helical and disordered loop conformations that interchange on a time scale of minutes. When incorporated around the edge of the smaller HDL particle, the increase in packing density of the two apoA-I molecules forces about 20% more residues out of direct contact with the phospholipid molecules to form disordered loops, and these are the same segments that form loops in the lipid-free state. The region of disc-associated apoA-I that binds the lecithin-cholesterol acyltransferase enzyme is well structured and not a protruding unstructured loop as reported by others.atherosclerosis | amphipathic α-helix | protein secondary structure T here is great interest in understanding the structure-function relationships of high-density lipoprotein (HDL) because of its important antiatherogenic properties. Because high-resolution structures of HDL microemulsion particles cannot be obtained by current X-ray crystallography and NMR methods, alternative biophysical approaches have been used to characterize various subspecies of HDL. Structural models that show the general lipid and protein organization in HDL particles are now available (for reviews, see refs. 1-6). To derive detailed understanding at the molecular level of how HDL functions in cholesterol transport (7) and in reducing the incidence of premature cardiovascular disease (8), higher-resolution structural information is required.Reconstituted discoidal HDL particles that are models of nascent HDL (9) created by the interaction of apolipoprotein (apo) A-I (3, 10), the principal protein of HDL, with the cellsurface ATP binding cassette transporter (ABCA1) (11, 12) have received a great deal of attention. A major advantage of model particles is the possibility of obtaining preparations that are sufficiently homogeneous for detailed structural investigation. The structure of a discoidal HDL particle (approximately 10 nm hydrodynamic diameter) comprising a 16...

show abstract

“…Moreover, the increasing EPA-PC contents in rHDL decreased the particle size, suggesting that EPA-rich rHDL forms saddle-shaped, minimal surface particles in high-fluidity membranes, which is consistent with the findings from a previous study. 23 There are possible mechanisms by which EPA-PC decreases rHDL particle size. Because EPA-PC increases membrane fluidity, the lipid bilayer in EPA-rich rHDL forms a saddle surface, which results in smaller rHDL particles.…”

Section: Discussionmentioning

confidence: 99%