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°, and the virion disrupted at 69°. 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.
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