Yasuda J, Hunter E
Department of Microbiology, University of Alabama at Birmingham, 35294, USA.
J Virol. 1998 May;72(5):4095-103. doi: 10.1128/JVI.72.5.4095-4103.1998.
Virus assembly represents one of the last steps in the retrovirus life cycle. During this process, Gag polyproteins assemble at specific sites within the cell to form viral capsids and induce membrane extrusion (viral budding) either as assembly progresses (type C virus) or following formation of a complete capsid (type B and type D viruses). Finally, the membrane must undergo a fusion event to pinch off the particle in order to release a complete enveloped virion. Structural elements within the MA region of the Gag polyprotein define the route taken to the plasma membrane and direct the process of virus budding. Results presented here suggest that a distinct region of Gag is necessary for virus release. The pp24 and pp16 proteins of the type D retrovirus Mason-Pfizer monkey virus (M-PMV) are phosphoproteins that are encoded in the gag gene of the virus. The pp16 protein is a C-terminally located cleavage product of pp24 and contains a proline-rich motif (PPPY) that is conserved among the Gag proteins of a wide variety of retroviruses. By performing a functional analysis of this coding region with deletion mutants, we have shown that the pp16 protein is dispensable for capsid assembly but essential for virion release. Moreover, additional experiments indicated that the virus release function of pp16 was abolished by the deletion of only the PPPY motif and could be restored when this motif alone was reinserted into a Gag polyprotein lacking the entire pp16 domain. Single-amino-acid substitutions for any of the residues within this motif confer a similar virion release-defective phenotype. It is unlikely that the function of the proline-rich motif is simply to inhibit premature activation of protease, since the PPPY deletion blocked virion release in the context of a protease-defective provirus. These results demonstrate that in type D retroviruses a PPPY motif plays a key role in a late stage of virus budding that is independent of and occurs prior to virion maturation.
病毒组装是逆转录病毒生命周期的最后步骤之一。在此过程中，Gag多聚蛋白在细胞内特定位点组装形成病毒衣壳，并在组装进行时（C型病毒）或完整衣壳形成后（B型和D型病毒）诱导膜挤出（病毒出芽）。最后，膜必须经历融合事件以掐断颗粒，从而释放完整的包膜病毒粒子。Gag多聚蛋白MA区域内的结构元件确定了通向质膜的途径，并指导病毒出芽过程。本文给出的结果表明，Gag的一个独特区域对于病毒释放是必需的。D型逆转录病毒梅森 - 辉瑞猴病毒（M-PMV）的pp24和pp16蛋白是磷酸化蛋白，由病毒的gag基因编码。pp16蛋白是pp24的C末端切割产物，包含一个富含脯氨酸的基序（PPPY），在多种逆转录病毒的Gag蛋白中保守。通过用缺失突变体对该编码区域进行功能分析，我们表明pp16蛋白对于衣壳组装是可有可无的，但对于病毒粒子释放是必不可少的。此外，额外的实验表明，仅缺失PPPY基序就会消除pp16的病毒释放功能，而当该基序单独重新插入缺乏整个pp16结构域的Gag多聚蛋白中时，该功能可以恢复。该基序内任何一个残基的单氨基酸替换都赋予类似的病毒粒子释放缺陷表型。富含脯氨酸基序的功能不太可能仅仅是抑制蛋白酶的过早激活，因为PPPY缺失在蛋白酶缺陷型原病毒的背景下阻断了病毒粒子的释放。这些结果表明，在D型逆转录病毒中，PPPY基序在病毒出芽的后期发挥关键作用，该阶段独立于病毒粒子成熟且发生在病毒粒子成熟之前。
