SCANNING CALORIMETRIC STUDIES OF THE STABILITY OF TOBACCO MOSAIC-VIRUS AND AGGREGATES OF ITS COAT PROTEIN

The thermal denaturation of the common strain of a rod-shaped plant vi rus, tobacco mosaic virus, has been investigated by differential scann ing calorimetry, and compared to that of various aggregation states of its coat protein and to that of three other TMV strains. The state of the virions was monitored by electron microscopy and analytical ultra centrifugation. The observed endotherms could be analysed in terms of a stepwise dissociation of the virions. The transition temperatures of the three successive structural changes increased with decreasing pH, from pH = 8.0 to pH = 5.0, although the corresponding enthalpy change s did not vary appreciably with pH. TMV-HR showed a stronger pH depend ence of the transition temperatures than the other strains, probably r eflecting the importance of the changes in affecting the charged amino acids of its coat protein. The first step of the dissociation, which correlates with the breaking up of the virions into three or four shor ter rods, implies a conformational change of the particle that may be related to the first step of the in situ decapsidation of TMV.