Larson Steven B, Lucas Robert W, McPherson Alexander
Department of Molecular Biology and Biochemistry, University of California, Irvine, CA 92697-3900, USA.
J Mol Biol. 2005 Feb 25;346(3):815-31. doi: 10.1016/j.jmb.2004.12.015. Epub 2005 Jan 12.
T=1 icosahedral particles of amino terminally truncated brome mosaic virus (BMV) protein were created by treatment of the wild-type T=3 virus with 1M CaCl2 and crystallized from sodium malonate. Diffraction data were collected from frozen crystals to beyond 2.9 A resolution and the structure determined by molecular replacement and phase extension. The particles are composed of pentameric capsomeres from the wild-type virions which have reoriented with respect to the original particle pentameric axes by rotations of 37 degrees , and formed tenuous interactions with one another, principally through conformationally altered C-terminal polypeptides. Otherwise, the pentamers are virtually superimposable upon those of the original T=3 BMV particles. The T=1 particles, in the crystals, are not perfect icosahedra, but deviate slightly from exact symmetry, possibly due to packing interactions. This suggests that the T=1 particles are deformable, which is consistent with the loose arrangement of pentamers and latticework of holes that penetrate the surface. Atomic force microscopy showed that the T=3 to T=1 transition could occur by shedding of hexameric capsomeres and restructuring of remaining pentamers accompanied by direct condensation. Knowledge of the structures of the BMV wild-type and T=1 particles now permit us to propose a tentative model for that process. A comparison of the BMV T=1 particles was made with the reassembled T=1 particles produced from the coat protein of trypsin treated alfalfa mosaic virus (AlMV), another bromovirus. There is little resemblance between the two particles. The BMV particle, with a maximum diameter of 195 A, is made from distinctive pentameric capsomeres with large holes along the 3-fold axis, while the AlMV particle, of approximate maximum diameter 220 A, has subunits closely packed around the 3-fold axis, large holes along the 5-fold axis, and few contacts within pentamers. In both particles crucial linkages are made about icosahedral dyads.
通过用1M氯化钙处理野生型T=3病毒，制备了氨基末端截短的雀麦花叶病毒（BMV）蛋白的T=1二十面体颗粒，并从丙二酸钠中结晶出来。从冷冻晶体收集衍射数据，分辨率超过2.9 Å，并通过分子置换和相位扩展确定结构。这些颗粒由野生型病毒粒子的五聚体壳粒组成，它们相对于原始颗粒的五聚体轴通过37度的旋转重新定向，并主要通过构象改变的C末端多肽相互形成微弱的相互作用。否则，这些五聚体实际上与原始T=3 BMV颗粒的五聚体重叠。晶体中的T=1颗粒不是完美的二十面体，而是略微偏离精确的对称性，这可能是由于堆积相互作用。这表明T=1颗粒是可变形的，这与五聚体的松散排列和穿透表面的孔的晶格结构一致。原子力显微镜显示，T=3到T=1的转变可能通过六聚体壳粒的脱落和剩余五聚体的重组并伴有直接凝聚而发生。现在，了解BMV野生型和T=1颗粒的结构使我们能够提出该过程的初步模型。将BMV T=1颗粒与由胰蛋白酶处理的苜蓿花叶病毒（AlMV，另一种雀麦病毒）的外壳蛋白产生的重组T=1颗粒进行了比较。这两种颗粒几乎没有相似之处。最大直径为195 Å的BMV颗粒由独特的五聚体壳粒制成，沿三重轴有大洞，而最大直径约为220 Å的AlMV颗粒，其亚基沿三重轴紧密堆积，沿五重轴有大洞，五聚体内的接触很少。在这两种颗粒中，关键的连接都是围绕二十面体二分体形成的。
