Structure of AP205 Coat Protein Reveals Circular Permutation in ssRNA Bacteriophages

•Structure of phage AP205 virus-like particles solved by combined X-ray, solid state NMR, and cryo-EM studies•AP205 displays a circular permutation compared to other phage coat proteins.•Coat protein termini are very surface exposed and suitable for fusions.•Intersubunit disulfide bonds are important for particle assembly and stability.

AP205 is a single-stranded RNA bacteriophage that has a coat protein sequence not similar to any other known single-stranded RNA phage. Here, we report an atomic-resolution model of the AP205 virus-like particle based on a crystal structure of an unassembled coat protein dimer and a cryo-electron microscopy reconstruction of the assembled particle, together with secondary structure information from site-specific solid-state NMR data. The AP205 coat protein dimer adopts the conserved Leviviridae coat protein fold except for the N-terminal region, which forms a beta-hairpin in the other known single-stranded RNA phages. AP205 has a similar structure at the same location formed by N- and C-terminal beta-strands, making it a circular permutant compared to the other coat proteins. The permutation moves the coat protein termini to the most surface-exposed part of the assembled particle, which explains its increased tolerance to long N- and C-terminal fusions.

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