Unusual self-assembly properties of Norovirus Newbury2 virus-like particles

In the Caliciviridae family of nonenveloped, positive-stranded RNA viruses, Noroviruses are major causes of human and animal gastroenteritis worldwide. The Norovirus T=3 icosahedral capsid is made of 180 copies of the VP1 protein, as exemplified in the crystal structure of the virus-like particle (VLP) of the human Norwalk virus (NV). It was previously shown that the ca 40-nm recombinant NV VLP can be disassembled and reassembled in vitro. Here we report on the disassembly and self-assembly properties for the related (VP1 sequence identity of 50%) bovine Newbury2 Norovirus (NB2) VLP. Using a panel of biophysical techniques, we show that while the NB2 VLP displays disassembly properties similar to the NV VLP, NB2-VP1 shows remarkable self-assembly properties heretofore unreported for NV-VP1 or any other calicivirus capsid protein. These properties include the capabilities of self-assembling not only into regular T=3 capsids but also into larger VLP (up to 76 nm in diameter) and of tolerating substitution of the spike domain for that of a distantly related Calicivirus. In conditions favoring the natural, T=3 capsid, NB2-VP1 reproducibly assembles by an apparent two-phase process. Our results establish a robust new system with which to probe the dynamics of viral capsid self-assembly.