eLife Assessment: Fine-tuning of outer membrane–peptidoglycan tethering by the redox-active lipoprotein LppB from Salmonella enterica

Abstract

The bacterial envelope is an essential compartment for survival, acting as a protective and permeability barrier. In diderm bacteria, it comprises an inner membrane, a thin peptidoglycan layer, and an outer membrane. A stable outer membrane-peptidoglycan connection is crucial for envelope integrity, preventing outer membrane detachment and enabling the generation of periplasmic turgor that counterbalances cytoplasmic pressure, thereby limiting susceptibility to antibiotics and environmental stressors. In Escherichia coli, the major lipoprotein Lpp (Braun’s lipoprotein) serves as the only covalent linker between the outer membrane and peptidoglycan, through its unique C-terminal lysine residue. Homologs of Lpp are widely conserved among Enterobacteriaceae, including Salmonella enterica serovar Typhimurium, which encodes LppA and LppB. While Lpp and LppA function similarly, the role of LppB remains unclear. Using E. coli as a surrogate model, we investigated the structural and functional properties of Salmonella LppB. Here, we demonstrate that LppB forms disulfide-linked dimers via its unique C-terminal cysteine and crosslinks to peptidoglycan through its terminal lysine, albeit with significantly lower efficiency than Lpp. Notably, LppB interacts with Lpp to form heterotrimers, thereby reducing overall Lpp-peptidoglycan crosslinking. These findings suggest that LppB modulates outer membrane-peptidoglycan tethering by fine-tuning the bacterial envelope architecture in response to environmental conditions. Given the established link between outer membrane-peptidoglycan uncoupling and outer membrane vesicles (OMV) formation, a key mechanism in bacterial pathogenicity, our results suggest a model in which LppB regulates outer membrane-peptidoglycan attachment, potentially influencing OMV production. This study highlights a modulating role for LppB in envelope remodeling, providing new insights into how Salmonella adapts its envelope structure in response to environmental cues to enhance fitness and virulence.