Virudachalam R, Low P S, Argos P, Markley J L
Virology. 1985 Oct 30;146(2):213-20. doi: 10.1016/0042-6822(85)90005-4.
In the differential scanning calorimetry (DSC) scans of turnip yellow mosaic virus (TYMV) or its capsid a single endotherm was observed. The endotherm was attributed to disruption of the virion or capsid structure with accompanying protein denaturation. At pH 4.5 the thermal stabilities of the TYMV virion and capsid were similar. With increasing pH, the capsid stability increased while the virion stability decreased. At neutral pH the capsid disrupted at 83.5 degrees, and the virion disrupted at 69 degrees. Our results suggest that packaging of viral RNA in the TYMV capsid imparts instability. The pHmid for disruption of the TYMV capsid is 5.7, which is in the pKa range expected for histidine side chains. Hence repulsive interactions involving one or more of the three histidines of the TYMV coat protein may explain the decreased stability of the TYMV capsid at low pH. This conclusion is supported further by the observation that belladonna mottle virus (BDMV) capsid (BDMV and TYMV belong to the tymo virus group), which contains no histidine in its coat protein, did not exhibit pH-dependent stability. The size of the cooperative unit in the disruption of TYMV capsid was estimated to be approximately that of a dimer of the coat protein, at pH 7.0, but a larger oligomer at low pH. Several reports implicate pH-dependent protein-RNA interactions with a pHmid near 7 as important in stabilizing tymovirus virions. Both DSC and 31P nuclear magnetic resonance linewidth analyses of the TYMV virion showed a transition midpoint at pH 7.0.
在芜菁黄花叶病毒（TYMV）或其衣壳的差示扫描量热法（DSC）扫描中观察到一个吸热峰。该吸热峰归因于病毒粒子或衣壳结构的破坏以及伴随的蛋白质变性。在pH 4.5时，TYMV病毒粒子和衣壳的热稳定性相似。随着pH值升高，衣壳稳定性增加而病毒粒子稳定性降低。在中性pH值下，衣壳在83.5摄氏度时破坏，病毒粒子在69摄氏度时破坏。我们的结果表明，TYMV衣壳中病毒RNA的包装导致不稳定性。TYMV衣壳破坏的pHmid为5.7，这在组氨酸侧链预期的pKa范围内。因此，涉及TYMV外壳蛋白三个组氨酸中一个或多个的排斥相互作用可能解释了TYMV衣壳在低pH值下稳定性降低的原因。颠茄斑驳病毒（BDMV）衣壳（BDMV和TYMV属于番茄花叶病毒组）在其外壳蛋白中不含组氨酸，未表现出pH依赖性稳定性，这一观察结果进一步支持了这一结论。在pH 7.0时，估计TYMV衣壳破坏中协同单元的大小约为外壳蛋白二聚体的大小，但在低pH值下为更大的寡聚体。几份报告表明，pH依赖性的蛋白质-RNA相互作用，其pHmid接近7，对稳定番茄花叶病毒粒子很重要。TYMV病毒粒子的DSC和31P核磁共振线宽分析均显示在pH 7.0时有一个转变中点。
