Towards an Efficient RNA Delivery System - Structural Insights into the Infection Cycle of ssRNA Phages

Abstract

Single-stranded RNA bacteriophages (ssRNA phages) infect Gram-negative bacteria via a single maturation protein (Mat), which attaches to a retractile pilus of the host. Using cryo-electron microscopy, we studied multiple steps that are essential for the phage infection cycle, including packaging, lysis and adsorption. Recently, we solved structures of the ssRNA phage MS2 in complex with the Escherichia coli F-pilus, showing a network of hydrophobic and electrostatic interactions at the Mat-pilus interface. Moreover, binding of the pilus induces slight orientational variations of the Mat relative to the rest of the phage capsid, priming the Mat-connected genomic RNA (gRNA) for its release from the virions. The exposed tip of the attached Mat points opposite to the direction of the pilus retraction, which may facilitate the translocation of the gRNA from the capsid into the host cytosol. In addition, our structures, for the first time, determine the orientation of the assembled F-pilin subunits relative to the cell envelope, providing new insights into the F-like type IV secretion systems.