Greene B, King J
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139, USA.
Virology. 1996 Nov 1;225(1):82-96. doi: 10.1006/viro.1996.0577.
Assembly of the icosahedral shells of the dsDNA bacteriophages, herpesviruses, and adenoviruses requires proteins not found in the mature virion, termed scaffolding proteins. The bacteriophage P22 precursor procapsid contains approximately 300 scaffolding molecules within a shell composed of 420 coat protein subunits. Though nonsense mutants are common, few mutants affecting the functions of the scaffolding protein have been recovered. We report here the isolation and characterization of new missense mutants unable to form infectious virions under restrictive conditions. These mutant scaffolding subunits were competent for protein folding and capsid assembly under restrictive conditions. Two mutants were defective in assembly into the procapsid of the portal complex, which serves as the channel through which DNA is packaged. These mutations may identify a region of the scaffolding protein required for interaction with the portal subunits. Two mutants in a different region of the sequence were impaired in scaffolding release from the procapsid both in vivo and in vitro. These mutations may identify a new domain required for scaffolding release. Scaffolding release appeared to be required for capsid expansion; in turn, scaffolding release seemed to depend upon the presence of a portal. This may help to order the pathway of events in phage maturation.
双链DNA噬菌体、疱疹病毒和腺病毒的二十面体衣壳组装需要成熟病毒粒子中不存在的蛋白质，即支架蛋白。噬菌体P22前衣壳在由420个衣壳蛋白亚基组成的外壳内含有大约300个支架分子。虽然无义突变很常见，但很少有影响支架蛋白功能的突变体被分离出来。我们在此报告新错义突变体的分离和表征，这些突变体在限制条件下无法形成感染性病毒粒子。这些突变的支架亚基在限制条件下能够进行蛋白质折叠和衣壳组装。两个突变体在组装成作为DNA包装通道的门户复合物前衣壳时存在缺陷。这些突变可能确定了支架蛋白与门户亚基相互作用所需的区域。序列中不同区域的两个突变体在体内和体外从前衣壳释放支架的过程中均受损。这些突变可能确定了支架释放所需的一个新结构域。支架释放似乎是衣壳扩张所必需的；反过来，支架释放似乎依赖于门户的存在。这可能有助于梳理噬菌体成熟过程中的事件顺序。
