Department of Medicine and Atherosclerosis Research Unit, University of Alabama, Birmingham, Alabama 35294, USA.
J Biol Chem. 2010 Dec 24;285(52):41161-71. doi: 10.1074/jbc.M110.187799. Epub 2010 Oct 25.
For several decades, the standard model for high density lipoprotein (HDL) particles reconstituted from apolipoprotein A-I (apoA-I) and phospholipid (apoA-I/HDL) has been a discoidal particle ∼100 Å in diameter and the thickness of a phospholipid bilayer. Recently, Wu et al. (Wu, Z., Gogonea, V., Lee, X., Wagner, M. A., Li, X. M., Huang, Y., Undurti, A., May, R. P., Haertlein, M., Moulin, M., Gutsche, I., Zaccai, G., Didonato, J. A., and Hazen, S. L. (2009) J. Biol. Chem. 284, 36605-36619) used small angle neutron scattering to develop a new model they termed double superhelix (DSH) apoA-I that is dramatically different from the standard model. Their model possesses an open helical shape that wraps around a prolate ellipsoidal type I hexagonal lyotropic liquid crystalline phase. Here, we used three independent approaches, molecular dynamics, EM tomography, and fluorescence resonance energy transfer spectroscopy (FRET) to assess the validity of the DSH model. (i) By using molecular dynamics, two different approaches, all-atom simulated annealing and coarse-grained simulation, show that initial ellipsoidal DSH particles rapidly collapse to discoidal bilayer structures. These results suggest that, compatible with current knowledge of lipid phase diagrams, apoA-I cannot stabilize hexagonal I phase particles of phospholipid. (ii) By using EM, two different approaches, negative stain and cryo-EM tomography, show that reconstituted apoA-I/HDL particles are discoidal in shape. (iii) By using FRET, reconstituted apoA-I/HDL particles show a 28-34-Å intermolecular separation between terminal domain residues 40 and 240, a distance that is incompatible with the dimensions of the DSH model. Therefore, we suggest that, although novel, the DSH model is energetically unfavorable and not likely to be correct. Rather, we conclude that all evidence supports the likelihood that reconstituted apoA-I/HDL particles, in general, are discoidal in shape.
几十年来，由载脂蛋白 A-I（apoA-I）和磷脂组成的高密度脂蛋白（HDL）颗粒的标准模型一直是一种直径约 100Å、磷脂双层厚度的盘状颗粒。最近，Wu 等人（Wu，Z.，Gogonea，V.，Lee，X.，Wagner，M. A.，Li，X. M.，Huang，Y.，Undurti，A.，May，R. P.，Haertlein，M.，Moulin，M.，Gutsche，I.，Zaccai，G.，Didonato，J. A.，and Hazen，S. L.（2009）J. Biol. Chem. 284，36605-36619）使用小角度中子散射开发了一种新模型，他们称之为双超螺旋（DSH）apoA-I，与标准模型有很大的不同。他们的模型具有开放的螺旋形状，围绕拉长的 I 型六方各向同性液晶相包裹。在这里，我们使用三种独立的方法，分子动力学、电子显微镜断层扫描和荧光共振能量转移光谱（FRET）来评估 DSH 模型的有效性。（i）通过使用分子动力学，两种不同的方法，全原子模拟退火和粗粒化模拟，表明初始的椭圆形 DSH 颗粒迅速坍塌成盘状双层结构。这些结果表明，与当前的脂质相图知识一致，apoA-I 不能稳定磷脂的六方 I 相颗粒。（ii）通过使用 EM，两种不同的方法，负染色和 cryo-EM 断层扫描，表明重组 apoA-I/HDL 颗粒呈盘状。（iii）通过使用 FRET，重组 apoA-I/HDL 颗粒显示末端结构域残基 40 和 240 之间的 28-34Å 分子间距离，这与 DSH 模型的尺寸不兼容。因此，我们认为，尽管新颖，但 DSH 模型在能量上是不利的，不太可能是正确的。相反，我们得出结论，所有证据都支持这样一种可能性，即重组 apoA-I/HDL 颗粒通常呈盘状。
