Self-assembly of brome mosaic virus protein into capsids. Initial and final states of aggregation

The pH and ionic strength dependence of the states of aggregation of brome mosaic virus protein has been investigated by small angle neutron scattering, quasielastic light-scattering, analytical centrifugation and electron microscopy. At pH above neutrality, protein oligomers are found in dynamical equilibrium, comprising monomers, dimers and aggregates of higher molecular weight. By lowering the pH, capsids assemble spontaneously with dimensions in solution which depend on ionic strength. If formed by dialysis, they contain 180 monomers, but are 30 A larger in diameter than the native virus. If formed by pH-jump, they contain less monomers: the deficiency decreases with decreasing the final pH and the initial protein concentration. Upon dehydration for electron microscopy, capsids contract by 10%.