An SPFH Protein Couples Membrane Stress to Differentiation in Bacillus subtilis

Bacillus subtilis adapts to fluctuating environmental stress, such as membrane perturbation or alkaline conditions, using membrane-associated regulatory complexes. Here, we rename the previously termed pspA-ydjGHI operon to pspA-samGHI (for starvation and motility) to reflect its functional roles. The membrane proteins SamG and SamH recruit SamI, a cytosolic SPFH (Stomatin/Prohibitin/Flotillin/HflK-C) protein that coordinates the recruitment of PspA, an ESCRT-III homologue from a protein family involved in membrane repair and remodelling. Here we show that SamGHI impacts developmental processes, allowing proper motility, secretion, and colony biofilm formation in the laboratory B. subtilis strain WT168. Loss of SamI (ΔsamI/ΔydjI) leads to unbalanced SamH activity, resulting in global transcriptional changes reminiscent of starvation situations. This, in turn, reduces secretion of the matrix protein BslA, preventing colony biofilm formation. Deletion of samH in combination with ΔsamI restores colony morphology, while ΔpspA mutants form normal colonies, indicating that PspA is dispensable for the developmental phenotype. Our findings reveal that, beyond its role in maintaining membrane integrity, SamGHI influences the cellular physiological state and thereby impacts multicellular development.