da Poian A T, Oliveira A C, Silva J L
Departamento de Bioquimica, Universidade Federal do Rio de Janeiro, Brazil.
Biochemistry. 1995 Feb 28;34(8):2672-7. doi: 10.1021/bi00008a034.
Assembly of icosahedral viruses is not completely understood at the molecular level. The main puzzle is to answer how chemically identical protein subunits take up unique positionally dependent conformations during the process of assembly. The stability of the ribonucleoprotein particles of cowpea mosaic virus (CPMV) to pressures and subzero temperatures has been studied. At room temperature, reversible pressure denaturation of CPMV is obtained only in the presence of 5.0 M urea. On the other hand, when the temperature is decreased to -15 degrees C, the ribonucleoprotein components denature, at 2.5 kbar, in the presence of 1.0 M urea. At temperatures close to -20 degrees C, denaturation is obtained even in the absence of urea. Whereas the denaturation promoted by pressure and urea at room temperature is reversible, virus particles denatured when the temperature is decreased under pressure cannot reassemble. Bis-ANS binding data suggest that this irreversibility may be related to protein release from RNA, which probably does not occur under denaturating conditions at room temperature. The contributions of enthalpy (delta H*) and entropy (delta S*) for the free energy of association of CPMV are calculated from the cold denaturation curves under pressure. The entropy change is positive and large, making the assembly of ribonucleoprotein components an entropy-driven process, suggesting that the burial of nonpolar side chains during the process of assembly is the structural foundation for CPMV assembly.
二十面体病毒的组装在分子水平上尚未完全被理解。主要的谜题在于回答在组装过程中，化学性质相同的蛋白质亚基是如何占据独特的位置依赖性构象的。人们已经研究了豇豆花叶病毒（CPMV）核糖核蛋白颗粒对压力和零下温度的稳定性。在室温下，只有在5.0 M尿素存在的情况下，CPMV才会发生可逆的压力变性。另一方面，当温度降至-15℃时，在1.0 M尿素存在的情况下，核糖核蛋白成分在2.5千巴压力下会变性。在接近-20℃的温度下，即使没有尿素也会发生变性。虽然在室温下由压力和尿素促进的变性是可逆的，但在压力下温度降低时变性的病毒颗粒无法重新组装。双-ANS结合数据表明，这种不可逆性可能与蛋白质从RNA上的释放有关，而在室温下的变性条件下这种释放可能不会发生。根据压力下的冷变性曲线计算了CPMV缔合自由能的焓变（ΔH*）和熵变（ΔS*）。熵变是正的且很大，这使得核糖核蛋白成分的组装成为一个熵驱动的过程，表明在组装过程中非极性侧链的埋藏是CPMV组装的结构基础。
