A defining feature of Rift Valley fever virus (RVFV) is the incorporation of the NSs protein into large filamentous assemblies inside infected nuclei R. Swanepoel, N. K. Blackburn, J. Gen. Virol. 34 , 557–561 (1977)., as judged from fixed specimens. To gain insight into the 3D structure of NSs filaments within live-cell nuclei, we used genetic-code expansion (GCE) to incorporate trans-cyclooct-2-en-L-lysine into the protein. This enabled site-specific fluorescent labeling with tetrazine dyes for live-cell structured illumination microscopy (SIM). Our superresolved images revealed the complete native architecture of NSs filaments as a micron-scale polygon web of fibers with discrete domain characteristics, overturning previous assumptions of simple linear filaments. Parallel experiments on fixed RVFV-infected cells confirmed that native NSs filaments also display this morphology. Overall, our 3D-SIM analysis reveals distinct structural plasticity within NSs filaments, establishing a quantitative structure–function relationship that support the importance of polygon organization for NSs filament function during RVFV infection.
Dunlop et al. (Mon,) studied this question.