The Staphylococcus aureus Pathogenicity Island 1 Protein gp6 Functions as an Internal Scaffold during Capsid Size Determination

Staphylococcus aureus pathogenicity island 1 (SaPI1) is a mobile genetic element that carries genes for several superantigen toxins. SaPI1 is normally stably integrated into the host genome but can become mobilized by “helper” bacteriophage 80α, leading to the packaging of SaPI1 genomes into phage-like transducing particles that are composed of structural proteins supplied by the helper phage but having smaller capsids. We show that the SaPI1-encoded protein gp6 is necessary for efficient formation of small capsids. The NMR structure of gp6 reveals a dimeric protein with a helix–loop–helix motif similar to that of bacteriophage scaffolding proteins. The gp6 dimer matches internal densities that bridge capsid subunits in cryo-electron microscopy reconstructions of SaPI1 procapsids, suggesting that gp6 acts as an internal scaffolding protein in capsid size determination.

► SaPI1 gp6 is required for the assembly of T = 4 capsids from phage 80α capsid protein. ► SaPI1 gp6 forms a dimer that resembles the scaffolding protein of phage ϕ29. ► gp6 dimers bridge capsid protein capsomers on the inside of SaPI1 procapsids. ► gp6 functions as an internal scaffolding protein during SaPI1 procapsid assembly.