BamA self-insertion drives inner-to-outer membrane remodelling in diderm Firmicutes

Area of Science:

• Microbiology
• Bacterial Cell Biology
• Protein Biochemistry

Background:

• Endospore formation is crucial for Firmicutes survival.
• Diderm bacteria like Acetonema longum possess unique outer spore membranes (OsM) that become functional outer membranes (OM) upon germination.
• The biogenesis of the OM during germination in diderm bacteria remains poorly understood.

Purpose of the Study:

• To investigate the mechanism of outer membrane (OM) biogenesis during spore germination in diderm Firmicutes.
• To identify novel proteins involved in OM remodeling and function.
• To elucidate the roles of BamA, SonA, and SonB in OM formation.

Main Methods:

• Comparative proteomics and gene expression profiling across different growth stages.
• Analysis of outer membrane protein (OMP) localization in vegetative, germinating, and mature spores.
• In vitro liposome insertion assays for key proteins.

Main Results:

• Outer membrane proteins (OMPs), including BamA and LptD, are absent in mature spores, indicating OM biogenesis occurs without pre-existing OMPs.
• Two novel proteins, SonA (a beta-barrel OMP) and SonB (a predicted OM lipoprotein), were identified and found to be co-expressed with BamA in a conserved operon.
• BamA and SonA demonstrated spontaneous insertion into liposomes, suggesting a self-assembly mechanism for OM formation.

Conclusions:

• OM biogenesis during germination in diderm Firmicutes is an active process involving de novo synthesis and assembly of OMPs.
• SonA and SonB are key players in the transformation of the OsM into a functional OM.
• The findings support a model where BamA and/or SonA initiate OM formation through self-insertion into the OsM.