Abstract
Staphylococcus aureus pathogenicity islands (SaPIs), such as SaPI1, exploit specific helper bacteriophages, like 80α, for their high frequency mobilization, a process termed 'molecular piracy'. SaPI1 redirects the helper's assembly pathway to form small capsids that can only accommodate the smaller SaPI1 genome, but not a complete phage genome. SaPI1 encodes two proteins, CpmA and CpmB, that are responsible for this size redirection. We have determined the structures of the 80α and SaPI1 procapsids to near-atomic resolution by cryo-electron microscopy, and show that CpmB competes with the 80α scaffolding protein (SP) for a binding site on the capsid protein (CP), and works by altering the angle between capsomers. We probed these interactions genetically and identified second-site suppressors of lethal mutations in SP. Our structures show, for the first time, the detailed interactions between SP and CP in a bacteriophage, providing unique insights into macromolecular assembly processes.
Highlights
Staphylococcus aureus is an opportunistic pathogen sometimes associated with serious skin and soft tissue infections in humans and animals (Lowy, 1998)
For SaPI1, we previously demonstrated that this size redirection depended on the SaPI1-encoded proteins CpmA and CpmB (Poliakov et al, 2008; Dearborn et al, 2011; Damle et al, 2012), leading to the formation of a 45 nm diameter capsid with T = 4 icosahedral symmetry (Dearborn et al, 2011) (Figure 1)
Cryo-electron microscopy (EM) images of 80a and SaPI1 procapsids were collected on an FEI Titan Krios electron microscope equipped with a DE-20 direct electron detector (Figure 2A,B)
Summary
Staphylococcus aureus is an opportunistic pathogen sometimes associated with serious skin and soft tissue infections in humans and animals (Lowy, 1998). The Stl repressor detects the presence of a helper phage by interacting with a phage early lytic gene product, such as 80a Sri or the Dut dUTPase (Tormo-Mas et al, 2010; Hill and Dokland, 2016), leading to derepression, excision and replication of the SaPI, followed by packaging into transducing particles made by phage-encoded proteins (Figure 1). This strategy allows the SaPI to escape the phage-induced cell lysis and instead spread horizontally through the bacterial population. This study has shown, for the first time, the detailed interactions between a scaffolding protein and a capsid, and provides new insights into the assembly and size determination process for viruses and other macromolecular complexes
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