HIV出芽和自组装的模型，细胞膜的作用
A model of HIV budding and self-assembly, role of cell membrane
摘要：
从宿主细胞的质膜出芽是人类免疫缺陷病毒（HIV）生命周期中不可或缺的一步，HIV属于包膜RNA病毒（逆转录病毒）的大家族。与常规包膜病毒不同，逆转录病毒出芽与逆转录病毒蛋白亚基（Gags）在细胞膜上自组装成球形病毒衣壳{ \em同时}发生。在这种独特的出芽和组装机制的引导下，我们研究了逆转录病毒出芽的自由能分布，同时考虑到Gag-Gag吸引能和膜弹性能。我们发现，如果Gag-Gag吸引力很强，萌芽总是进行到完成。在出芽初期，部分出芽衣壳的天顶角$α$随时间增加，为$α\propto t^{1/3}$。然而，当Gag-Gag吸引力较弱时，就会出现部分萌芽的亚稳态。这些部分球形衣壳的天顶角由线性近似的$α_0\simeq(τ^2/κσ)^{1/4}$给出，其中$κ$和$σ$是膜的弯曲模量和表面张力，并且$τ$是与Gag-Gag吸引力的强度成比例的衣壳的线张力。在数值上，我们发现$α_0<0.3π$没有任何近似。使用实验参数，我们表明，艾滋病毒的出芽和组装总是在正常的生物条件下进行完成。另一方面，通过改变Gag-Gag相互作用强度或膜刚度，相对容易在完全出芽和部分出芽之间来回调节。我们的模型与观察包括HIV在内的逆转录病毒部分出芽的实验相当吻合。
英文摘要：
Budding from the plasma membrane of the host cell is an indispensable step in the life cycle of the Human Immunodeficiency Virus (HIV), which belongs to a large family of enveloped RNA viruses, retroviruses. Unlike regular enveloped viruses, retrovirus budding happens {\em concurrently} with the self-assembly of retrovirus protein subunits (Gags) into spherical virus capsids on the cell membrane. Led by this unique budding and assembly mechanism, we study the free energy profile of retrovirus budding, taking into account of the Gag-Gag attraction energy and the membrane elastic energy. We find that if the Gag-Gag attraction is strong, budding always proceeds to completion. During early stage of budding, the zenith angle of partial budded capsids, $α$, increases with time as $α\propto t^{1/3}$. However, when Gag-Gag attraction is weak, a metastable state of partial budding appears. The zenith angle of these partially spherical capsids is given by $α_0\simeq(τ^2/κσ)^{1/4}$ in a linear approximation, where $κ$ and $σ$ are the bending modulus and the surface tension of the membrane, and $τ$ is a line tension of the capsid proportional to the strength of Gag-Gag attraction. Numerically, we find $α_0<0.3π$ without any approximations. Using experimental parameters, we show that HIV budding and assembly always proceed to completion in normal biological conditions. On the other hand, by changing Gag-Gag interaction strength or membrane rigidity, it is relatively easy to tune it back and forth between complete budding and partial budding. Our model agrees reasonably well with experiments observing partial budding of retroviruses including HIV.
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