Résumé
Non-segmented, (-)RNA viruses cause serious human diseases. Human metapneumovirus (HMPV), an emerging pathogen of this order of viruses (Mononegavirales) is one of the main causes of respiratory tract illness in children. To help elucidate the assembly mechanism of the nucleocapsid (the viral RNA genome packaged by the nucleoprotein N) we present crystallographic structures of HMPV N in its assembled RNA-bound state and in a monomeric state, bound to the polymerase cofactor P. Our structures reveal molecular details of how P inhibits the self-assembly of N and how N transitions between the RNA-free and RNA-bound conformational state. Notably, we observe a role for the C-terminal extension of N in directly preventing premature uptake of RNA by folding into the RNA-binding cleft. Our structures suggest a common mechanism of how the growth of the nucleocapsid is orchestrated, and highlight an interaction site representing an important target for antivirals. DOI:
http://dx.doi.org/10.7554/eLife.12627.001 Human metapneumovirus (HMPV for short) is a major cause of infections of the airways and lungs, particularly in children, elderly individuals and people with weakened immune systems. As for all viruses, HMPV cannot survive on its own. Instead, it must invade and hijack cells in order to replicate its own genetic material and form new viruses. In HMPV, this genetic information is in the form of a strand of RNA, and is protected by a shell-like structure called a nucleocapsid. Drugs that disrupt the nucleocapsid may therefore help to kill the viruses and treat the illnesses that they cause. Nucleocapsids are built out of many copies of a protein called nucleoprotein, which binds to a strand of RNA. However, viral nucleocapsids can only be built from nucleoproteins that are bound to viral RNA. Potentially, nucleoproteins could instead bind to RNA belonging to the cells that HMPV infects and they would then be trapped in a dead-end state. To prevent this type of unproductive binding, before the nucleocapsid is formed the nucleoprotein is kept unassembled with the help of another protein called the polymerase cofactor. However, it was not clear exactly how the polymerase cofactor helps to maintain this unassembled state. Using techniques called cryo-electron microscopy and X-ray crystallography, Renner et al. studied the structures formed when nucleoproteins are either bound to RNA or are unassembled and bind to the polymerase cofactor. Comparing these structures revealed that RNA normally binds to a specific cleft in the nucleoprotein. However, when nucleoprotein is bound to the polymerase cofactor a portion of the nucleoprotein folds into this cleft instead, blocking the insertion of RNA. This prevents the nucleoprotein from associating with the wrong RNA, allowing the nucleoprotein to remain in an unassembled state until it is needed for the virus. Renner et al. also found that the interactions between the nucleoprotein and the polymerase cofactor of HMPV occur at sites that are also found in several other related viruses, such as Ebola. Targeting this common region could therefore be a good strategy for developing new antiviral drugs. DOI:
http://dx.doi.org/10.7554/eLife.12627.002