Eastman S W, Linial M L
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, USA.
J Virol. 2001 Aug;75(15):6857-64. doi: 10.1128/JVI.75.15.6857-6864.2001.
In contrast to all retroviruses but similar to the hepatitis B virus, foamy viruses (FV) require expression of the envelope protein for budding of intracellular capsids from the cell, suggesting a specific interaction between the Gag and Env proteins. Capsid assembly occurs in the cytoplasm of infected cells in a manner similar to that for the B- and D-type viruses; however, in contrast to these retroviruses, FV Gag lacks an N-terminal myristylation signal and capsids are not targeted to the plasma membrane (PM). We have found that mutation of an absolutely conserved arginine (Arg) residue at position 50 to alanine (R50A) of the simian foamy virus SFV cpz(hu) inhibits proper capsid assembly and abolishes viral budding even in the presence of the envelope (Env) glycoproteins. Particle assembly and extracellular release of virus can be restored to this mutant with the addition of an N-terminal Src myristylation signal (Myr-R50A), presumably by providing an alternate site for assembly to occur at the PM. In addition, the strict requirement of Env expression for capsid budding can be bypassed by addition of a PM-targeting signal to Gag. These results suggest that intracellular capsid assembly may be mediated by a signal akin to the cytoplasmic targeting and retention signal CTRS found in Mason-Pfizer monkey virus and that FV Gag has the inherent ability to assemble capsids at multiple sites like conventional retroviruses. The necessity of Env expression for particle egress is most probably due to the lack of a membrane-targeting signal within FV Gag to direct capsids to the PM for release and indicates that Gag-Env interactions are essential to drive particle budding.
与所有逆转录病毒不同，但与乙型肝炎病毒相似，泡沫病毒（FV）需要包膜蛋白表达才能使细胞内的衣壳从细胞中出芽，这表明Gag和Env蛋白之间存在特定相互作用。衣壳组装在受感染细胞的细胞质中以类似于B型和D型病毒的方式发生；然而，与这些逆转录病毒不同的是，FV Gag缺乏N端肉豆蔻酰化信号，衣壳也不会靶向质膜（PM）。我们发现，将猿猴泡沫病毒SFV cpz(hu)第50位绝对保守的精氨酸（Arg）残基突变为丙氨酸（R50A）会抑制衣壳的正常组装，即使在存在包膜（Env）糖蛋白的情况下也会消除病毒出芽。通过添加N端Src肉豆蔻酰化信号（Myr-R50A），该突变体的病毒颗粒组装和细胞外释放可以恢复，推测是通过在质膜上提供一个替代的组装位点。此外，通过向Gag添加质膜靶向信号可以绕过Env表达对衣壳出芽的严格要求。这些结果表明，细胞内衣壳组装可能由类似于在梅森-辉瑞猴病毒中发现的细胞质靶向和保留信号CTRS的信号介导，并且FV Gag具有像传统逆转录病毒一样在多个位点组装衣壳的内在能力。Env表达对于病毒颗粒释放的必要性很可能是由于FV Gag中缺乏将衣壳导向质膜以进行释放的膜靶向信号，这表明Gag-Env相互作用对于驱动病毒颗粒出芽至关重要。
