Pressure-induced dissociation of brome mosaic virus

Brome mosaic virus reversibly dissociates into subunits in the pressure range of 600 x 10(5) to 1600 x 10(5) Pa, as demonstrated by studies of the spectral shift of intrinsic fluorescence, of filtration chromatography and of electron microscopy of samples fixed under pressure. Smaller shell particles (T = 1) were detected as intermediates in the dissociation pathway. Dissociation was facilitated by decreasing the concentration, as expected for a multimolecular reaction. The estimated change in volume upon dissociation into 90 dimer particles was -2960 ml/mol. Large increases in the intrinsic fluorescence intensity and in the binding of bis(8-anilinonaphthalene-1-sulfonate) occurred at pressures higher than 1400 x 10(5) Pa. The pressure-dependence profile of the different spectral properties shifted to lower pressures when 5 mM-MgCl2 was included in the buffer or when the pH was raised from 5.5 to 5.9. When the pressure was progressively increased above 1400 x 10(5) Pa, a value that led to 75% dissociation, the capsid subunits lost the ability to reassociate into regular shells and only amorphous aggregates were formed after decompression, as evidenced by both electron microscopy and gel filtration chromatography. The formation of these random aggregates of brome mosaic virus can be explained by a conformational drift of the separated subunits, similar in nature to that found in simpler oligomeric proteins.