Structural Basis for Adsorption of Single-Stranded RNA Bacteriophages

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. In this thesis work, I explored interactions between two pairs of the ssRNA phage/pilus complexes from different genera: (1) the canonical model of ssRNA phage MS2 and Escherichia coli F-pilus, (2) ssRNA phage AP205 and Acinetobacter genomospecies 16. type IV pilus.

For the MS2/F-pilus complex, we displayed a hydrophobic and electrostatic interaction network on the Mat-pilus interface. Moreover, the binding of the pilus causes a slight change in the orientation of Mat relative to the rest of the phage capsid, which primes the release of the Mat-linked genomic RNA (gRNA). When attaching to F-pilus, the tip of the Mat points to the opposite direction of the pilus retraction. This may facilitate the transfer of the gRNA from the capsid into the host cytoplasm. In addition, our structures determine the orientation of the F-pilus relative to the cell envelope, providing insights into the F-like type IV secretion systems.

In terms of AP205/Type IV pilus interaction, I first solved the high-resolution asymmetric structure of AP205 and showed a unique Mat homodimer replace coat protein dimer at the two-fold axis. Interestingly, each Mat monomer adopts the same secondary structure, but a slightly different tertiary structure, which may be the result of adaption to RNA binding. 1% of particles assemble into T=4 particles with large diameter capsids.

We experimentally approved type IV pili as the true host receptor for ssRNA phage AP205 and solved a high-resolution structure of the type IV pili from Acinetobacter genomospecies 16. We presented an AP205/Type IV pilus structure in this dissertation work and showed that the Mat homodimer can adsorb to two pili simultaneously. We also discussed the 3D variability of the AP205/Type IV pili complex.