Gertsman Ilya, Gan Lu, Guttman Miklos, Lee Kelly, Speir Jeffrey A, Duda Robert L, Hendrix Roger W, Komives Elizabeth A, Johnson John E
Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037, USA.
Nature. 2009 Apr 2;458(7238):646-50. doi: 10.1038/nature07686. Epub 2009 Feb 8.
Lambda-like double-stranded (ds) DNA bacteriophage undergo massive conformational changes in their capsid shell during the packaging of their viral genomes. Capsid shells are complex organizations of hundreds of protein subunits that assemble into intricate quaternary complexes that ultimately are able to withstand over 50 atm of pressure during genome packaging. The extensive integration between subunits in capsids requires the formation of an intermediate complex, termed a procapsid, from which individual subunits can undergo the necessary refolding and structural rearrangements needed to transition to the more stable capsid. Although various mature capsids have been characterized at atomic resolution, no such procapsid structure is available for a dsDNA virus or bacteriophage. Here we present a procapsid X-ray structure at 3.65 A resolution, termed prohead II, of the lambda-like bacteriophage HK97, the mature capsid structure of which was previously solved to 3.44 A (ref. 2). A comparison of the two largely different capsid forms has unveiled an unprecedented expansion mechanism that describes the transition. Crystallographic and hydrogen/deuterium exchange data presented here demonstrate that the subunit tertiary structures are significantly different between the two states, with twisting and bending motions occurring in both helical and beta-sheet regions. We also identified subunit interactions at each three-fold axis of the capsid that are maintained throughout maturation. The interactions sustain capsid integrity during subunit refolding and provide a fixed hinge from which subunits undergo rotational and translational motions during maturation. Previously published calorimetric data of a closely related bacteriophage, P22, showed that capsid maturation was an exothermic process that resulted in a release of 90 kJ mol(-1) of energy. We propose that the major tertiary changes presented in this study reveal a structural basis for an exothermic maturation process probably present in many dsDNA bacteriophage and possibly viruses such as herpesvirus, which share the HK97 subunit fold.
类λ双链（ds）DNA噬菌体在病毒基因组包装过程中，其衣壳会发生巨大的构象变化。衣壳是由数百个蛋白质亚基组成的复杂结构，这些亚基组装成复杂的四级复合物，最终能够在基因组包装过程中承受超过50个大气压的压力。衣壳中亚基之间广泛的整合需要形成一种中间复合物，称为前衣壳，单个亚基可以从该复合物进行必要的重折叠和结构重排，以转变为更稳定的衣壳。尽管各种成熟衣壳已在原子分辨率下得到表征，但尚无dsDNA病毒或噬菌体的前衣壳结构。在此，我们展示了类λ噬菌体HK97的前衣壳X射线结构，分辨率为3.65 Å，称为前头部II，其成熟衣壳结构先前已解析至3.44 Å（参考文献2）。对这两种差异很大的衣壳形式的比较揭示了一种前所未有的扩展机制，该机制描述了这种转变。此处呈现的晶体学和氢/氘交换数据表明，两种状态下亚基的三级结构存在显著差异，在螺旋区和β折叠区均发生扭曲和弯曲运动。我们还确定了衣壳每个三重轴上的亚基相互作用，这些相互作用在整个成熟过程中得以维持。这些相互作用在亚基重折叠过程中维持衣壳的完整性，并提供一个固定的铰链，亚基在成熟过程中可从该铰链进行旋转和平移运动。先前发表的密切相关噬菌体P22的量热数据表明，衣壳成熟是一个放热过程，会释放90 kJ mol⁻¹的能量。我们提出，本研究中呈现的主要三级变化揭示了放热成熟过程的结构基础，这一过程可能存在于许多dsDNA噬菌体以及可能诸如疱疹病毒等具有HK97亚基折叠的病毒中。
