DNA-induced viral assembly studied in real-time by OT, AFS and AFM

The simian vacuolating virus (SV40) can efficiently infect a wide range of human cells, which makes this capsid of interest as a capsule for drug delivery. The icosahedral virus capsid is assembled from VP1 pentamers, and this assembly is known to be facilitated by the presence of dsDNA. However, real-time assembly of SV40, and viruses in general, is poorly understood limiting among other things our ability to manipulate capsid re-assembly in vitro for therapeutic purposes. Here, we show real time assembly of VP1 pentamers on dsDNA employing dual-trap optical tweezers (OT) and a new technology, acoustic force spectroscopy (AFS). Real-time shortening of the DNA and modulated stretching curves suggests the assembly of (intermediate) capsid-like structures. Unbinding can be studied by applying relatively low forces to the DNA over a longer time scale. To obtain visual information, atomic force microscopy (AFM) images were captured showing significant cluster formation on DNA supporting the idea that we observe partial assembly. These results reveal that we can indeed track the assembly of viral capsids and visualize intermediate structures. This in turn provides a window into the complex kinetics of viral capsid formation