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The stability of turnip yellow mosaic virus (TYMV) was investigated under pressure, using solution neutron small angle scattering. Dissociation products were characterized by analytical ultracentrifugation and electron microscopy. At pH 6.0, TYMV remained unaffected by pressure, up to 260 Megapascals (MPa), the highest pressure reached in these experiments. At pH 8.0, TYMV remained unaffected by pressure up to 160 MPa, but decapsidated irreversibly above 200 MPa, giving rise to more and more empty shells upon increasing pressure. The organization of these empty shells was similar to that of the capsid of native virions, apart from the presence of a hole corresponding to the loss of a group of 5-8 coat protein subunits, through which the RNA may have escaped. At variance with other small isometric viruses, the capsid of TYMV never dissociated under pressure into subunits or small aggregates of subunits. This exceptional behavior of TYMV is probably due to the importance of van der Waals contacts and hydrogen bonds in the stability of its capsid.

Turnip yellow mosaic virus (TYMV) is a small isometric plant virus which decapsidates by releasing its RNA through a hole in the capsid, leaving behind an empty shell [R. E. F. Matthews and J. Witz, (1985) Virology 144, 318±327]. Similar empty shells (artificial top component, ATC) can be obtained by submitting the virions to various treatments in vitro. We have used differential scanning calorimetry, analytical sedimentation, and electron microscopy to investigate the thermodenaturation of natural empty shells (NTC, natural top component) present in purified virus suspensions, and of several types of ATCs. ATCs divided in two major classes. Those obtained by alkaline titration, by the action of urea or butanol behaved as NTC: their thermograms contained only one peak corresponding to the irreversible dissociation of the shells and the denaturation of the coat protein. The temperature of this unique transition varied significantly with pH, from 71 8C at pH 4.5 to 84 8C at pH 8.5. The thermograms of ATCs obtained by freezing and thawing, or by the action of high pressure, contained two peaks: shells dissociated first into smaller protein aggregates at 57 8C (at pH 5.0) to 61 8C (at pH 8.5), which denatured at the temperature of the unique transition of NTC. Shells obtained by heating virions to 55 8C at pH 7.6, changed conformation after the release of the viral RNA, as upon continuous heating to 95 8C, their thermograms were similar to those of the shells obtained by freezing and thawing, whereas after purification they behaved like NTC. Structural implications of these observations are discussed.

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