Self-assembly of brome mosaic virus capsids: insights from shorter time-scale experiments

Abstract

An amended kinetic model for the self-assembly of empty capsids of brome mosaic virus is proposed. The model has been modified to account for a new feature in the assembly kinetics revealed by time-course light scattering experiments at higher temporal resolution than previously attempted. To be able to simulate the sharp takeoff from the initial lag phase to the growth phase in the kinetic curves, a monomer activation step was proposed.

Figure 1

An example DLS diagram during early stages of BMV capsid assembly. DLS was set for general fitting; 3 peaks were listed as default display setting, though there were only 2 components captured. The peak of ~7.22 nm represents protein dimers and the peak of 29.4 nm represents capsids. Notably, intermediates were barely observed. The diagram was recorded in 60 s at 16 °C and a concentration of 4.19 μM of protein dimers.

Figure 2

Turbidity curves of BMV capsids assembly at various temperatures and protein concentration of 9.4 μM of protein dimers. The capsid assembly rate decreases with temperature (a). Turbidity curves are characterized by an initial flat line followed by a rapid increase of the scattering intensity (b).

Figure 3

Simulated turbidity curves (black lines) with the models of sKL₃ (a), sKL₄ (b) and sKL0₃ (c), and the experimental data (colored) for BMV capsid assembly at 10 °C. On the right column, the characteristic initial flat lag phase followed by a sharp turning into the rapid growth zone is illustrated for the assembly with 11.1 μM of protein dimers; sKL₃ and sKL₄ fail to produce the feature but sKL0₃ does.

Figure 4

sKL₀₃ prediction of scattering contributions of different components for the BMV assembly with a protein concentration of 4.19 μM dimers at 10 °C. The intermediates only make an ignorable contribution; the scattering from capsid rises up between 10 and 20 s, corresponding to the time needed for monomer activation, and in a fair agreement with DLS observation (supplemental Figure 1, Supporting Information).

Figure 5

Noise analysis for the lag-phase takeoff ranges. (a) sKL₃ and sKL₀₃ modeling results together with the smoothed data (by average of 4 adjacent points, which reduces the time resolution to 40 ms). It is clear that sKL₃ modeling rises over the noise during that range whereas sKL₀₃ reproduces the sharp turning feature. (b) Local squared deviation averaged among an 8 s span of both modeling from the smoothed data. The sKL3 result shows significant larger values around the turning range.