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

ABSTRACT: Endospore formation in Firmicutes is a complex morphological process triggered by environmental stress. At the end of this process, a mature spore containing the condensed genome is released through lysis of the mother cell. The mature spore is encased by several protective layers, including an inner and an outer spore membrane (IsM and OsM, respectively), both derived from the inner membrane (IM) of the mother cell. While this process is well understood in monoderm bacteria like Bacillus subtilis, less is known about diderm spore-formers such as Acetonema longum, which retain and remodel their OsM into a true outer membrane (OM) during germination. Our results show that outer membrane proteins (OMPs), including the essential β-barrel Assembly Machinery (BAM) component BamA, are present in vegetative and germinating cells but absent from mature spores. These observations highlight an unprecedented mechanism of inner to outer membrane remodelling in the absence of BamA. Expression profiles of cells at different growth stages and phylogenomic analyses identified two novel proteins, co-expressed and in an operon with BamA, that were highly conserved among diderm Firmicutes: a β-barrel OMP containing three polypeptide transport-associated (POTRA) domains (SonA) and a putative OM lipoprotein (SonB). In vitro experiments further demonstrated that both BamA and SonA can self-insert into liposomes under physiological conditions. Together, we propose a model in which BamA and its functional partners, SonA and SonB, drive membrane remodelling during germination. The process is initiated by self-insertion of BamA, which then assists the folding and insertion of other OMPs, including LptD, progressively shifting the protein and lipid composition of the OsM to produce a true OM. Collectively, our findings provide new insights into the functional role of BamA in membrane remodelling during OM biogenesis.

ORGANISM(S): Acetonema longum

PROVIDER: GSE306945 | GEO | 2025/09/06

REPOSITORIES: GEO