Near-atomic structure of the PorKN rings, disulfide bonded to PorG and bound to Attachment Complexes, provide mechanistic insights into the type IX secretion system

Abstract

The Type IX Secretion System exports proteins across the outer membrane (OM) of bacteria in the Bacteroidota phylum, however, the mechanistic details remain unknown. Here, we present a [~]3.5[A] cryo-EM structure of the periplasmic rings comprising 32-33 subunits each of PorK and PorN. Additionally, we show the presence of a critical disulfide bond between PorK and the PorG OM protein that is essential for protein secretion and demonstrate that the Attachment Complexes bind to and are localized above the PorKN rings. Overall, each ring resembles a cogwheel with PorN forming cog-like projections on the periplasmic side and the flat surface of PorK orienting towards the OM. Given these results, we propose that the PorLM motor drives the rotation of the PorKN cogwheel together with PorG and associated Attachment Complexes, potentially providing the energy to complete protein secretion and the coordinated cell surface attachment of the secreted cargo.