Asymmetric envelope surface disposition of secreted protein YjbI controls bimodal antibiotic susceptibilities in C. crescentus

Cytoplasmic pentapeptide repeat proteins (PRPs) protect bacterial DNA gyrase from quinolone antibiotics. While some secreted PRPs are essential upon quinolone exposure, their role in the regulation of antibiotic resistance remains to be fully characterized. We show that a YjbI-type secreted PRP regulates antibiotic sensitivity, bimodally for small or large molecules, via modulation of the Caulobacter crescentus outer membrane (OM). YjbI silences two converging envelope-stress pathways that globally reprogram the OM proteome via TonB-dependent receptors (TBDRs), periplasmic proteases, and AcrAB-NodT, a multidrug efflux pump whose induction by small molecules and antibiotics is lethal to yjbI mutant cells. Loss of YjbI also confers sensitivity to vancomycin and bacitracin, two large peptidoglycan-targeting and zinc-binding antibiotics that permeate the outer membrane via the previously uncharacterized TBDR BugA and its orthologs. Zinc stress triggers rapid proteolytic removal of Yjbl, activates expression of TBDRs, including BugA, and ultimately leads to replenishment of YjbI. Molecular dynamics simulations and reactive thiol probing imply an asymmetric surface disposition of YjbI, explaining the differential accessibility of its conserved cysteine pairs that flank the quadrilateral β-helix. Taken together, our findings identify a role of YjbI as a cell surface-regulator of outer membrane composition and antibiotic sensitivity in a Gram-negative bacterium.