Intermolecular latency regulates the essential C-terminal signal peptidase and sortase of the secretion

Porphyromonas gingivalis is a keystone pathogen of the human dysbiotic oral microbiome that causes severe periodontitis. It employs a type-IX secretion system (T9SS) to shuttle proteins across the outer membrane (OM) for virulence. Uniquely, T9SS cargoes carry a C-terminal domain (CTD) as a secretion signal, which is cleaved and replaced with anionic lipopolysaccharide by transpeptidation for extracellular anchorage to the OM. Both reactions are carried out by PorU, the only known dual-function, C-terminal signal peptidase and sortase. PorU is itself secreted by the T9SS, but its CTD is not removed; instead, intact PorU combines with PorQ, PorV, and PorZ in the OM-inserted attachment complex. Herein, we revealed that PorU transits between active monomers and latent dimers and solved the crystal structure of the -260-kDa dimer. PorU has an elongated shape -130 angstrom in length and consists of seven domains. The first three form an intertwined N-terminal cluster likely engaged in substrate binding. They are followed by a gingipain-type catalytic domain (CD), two immunoglobulin-like domains (IGL), and the CTD. In the first IGL, a long latency j3-hairpin protrudes -30 angstrom from the surface to form an intermolecular j3-barrel with j3-strands from the symmetric CD, which is in a latent conformation. Homology modeling of the competent CD followed by in vivo validation through a cohort of mutant strains revealed that PorU is transported and functions as a monomer through a C690/H657 catalytic dyad. Thus, dimerization is an intermolecular mechanism for PorU regulation to prevent untimely activity until joining the attachment complex.

Automated summary

Porphyromonas gingivalis employs the T9SS system to transport proteins to the outer membrane, where PorU plays a key role in forming an attachment complex. PorU functions as both an active monomer and a latent dimer, with an elongated shape consisting of seven domains including a crucial C-terminal domain.