Structures of Qβ virions, virus-like particles, and the Qβ–MurA complex reveal internal coat proteins and the mechanism of host lysis

Significance Host lysis and virion assembly are essential processes during the infection cycle of single-stranded RNA (ssRNA) viruses. Using single-particle cryoelectron microscopy, we visualized how the ssRNA virus, Qβ, uses its single-molecule “tail protein,” A 2 , to inhibit MurA, a bacterial enzyme essential for cell wall biosynthesis, leading to lysis of the host cell. We also revealed an extra coat protein dimer, which instead of being a part of the viral capsid, is sequestered within the virion, binding to an RNA hairpin from a five-way junction in the genomic RNA. The same five-way junction also presents hairpins to bind A 2 and other coat protein dimers in the capsid, potentially supporting a nucleation event for virion assembly.