Kinetics of the in vitro Assembly of Brome Mosaic Virus Capsids
The protein subunits of brome mosaic virus can self-assemble in vitro, in the absence of RNA, to form capsidlike particles that are similar in size to the original virus capsid. The protein exists essentially as dimers at neutral pH. The assembly process is triggered by a decrease in pH. The kinetics of assembly were followed by using stopped-flow techniques coupled with neutron or x-ray small-angle scattering. Neutron scattering experiments were performed at the I.L.L, Grenoble, and x-ray studies at L.U.R.E., Orsay. Both the x-ray and neutron investigations showed that the assembly process starts rapidly: the intensity at the origin, I(O), achieves 60% of its maximal value in a few hundred milliseconds and 80% in a few seconds. Furthermore, analysis of the scattering curves suggests that aggregates similar in size to the native viral capsid are present after only 300 msec. Then, the process gradually slows down, as indicated by I(O), which continues to increase very slowly over a period of hours. Finally, the possible existence of intermediate species is discussed on the basis of our experimental data.