Heinz F X, Auer G, Stiasny K, Holzmann H, Mandl C, Guirakhoo F, Kunz C
Institute of Virology, University of Vienna, Austria.
Arch Virol Suppl. 1994;9:339-48. doi: 10.1007/978-3-7091-9326-6_34.
Viral membrane proteins play an important role in the assembly and disassembly of enveloped viruses. Oligomerization and proteolytic cleavage events are involved in controlling the functions of these proteins during virus entry and release. Using tick-borne encephalitis virus as a model we have studied the role of the flavivirus envelope proteins E and prM/M in these processes. Experiments with acidotropic agents provide evidence that the virus is taken up by receptor-mediated endocytosis and that the acidic pH in endosomes plays an important role for virus entry. The envelope glycoprotein E undergoes irreversible conformational changes at acidic pH, as indicated by the loss of several monoclonal antibody-defined epitopes, which coincide with the viral fusion activity in vitro. Sedimentation analysis reveals that these conformational changes lead to aggregation of virus particles, apparently by the exposure of hydrophobic sequence elements. None of these features are exhibited by immature virions containing E and prM rather than E and M. Detergent solubilization, sedimentation, and crosslinking experiments provide evidence that prM forms a complex with protein E which prevents the conformational changes necessary for fusion activity. The functional role of prM before its endoproteolytic cleavage by a cellular protease thus seems to be the protection of protein E from acid-inactivation during its passage through acidic trans Golgi vesicles in the course of virus release.
病毒膜蛋白在包膜病毒的组装和拆卸过程中发挥着重要作用。寡聚化和蛋白水解切割事件参与了病毒进入和释放过程中这些蛋白功能的调控。以蜱传脑炎病毒为模型，我们研究了黄病毒包膜蛋白E和prM/M在这些过程中的作用。使用亲酸性试剂进行的实验表明，病毒通过受体介导的内吞作用被摄取，并且内体中的酸性pH值对病毒进入起着重要作用。包膜糖蛋白E在酸性pH值下会发生不可逆的构象变化，这表现为几个单克隆抗体定义的表位的丧失，这与体外病毒融合活性相吻合。沉降分析表明，这些构象变化导致病毒颗粒聚集，显然是由于疏水序列元件的暴露。含有E和prM而非E和M的未成熟病毒粒子不表现出这些特征。去污剂溶解、沉降和交联实验表明，prM与蛋白E形成复合物，从而阻止了融合活性所需的构象变化。因此，在被细胞蛋白酶进行内切蛋白水解切割之前，prM的功能作用似乎是在病毒释放过程中保护蛋白E在通过酸性反式高尔基体囊泡时不被酸灭活。
