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 in membrane envelope stress signalling. The SamG-SamH membrane proteins recruit SamI, a cytosolic SPFH protein, which stabilizes focal membrane localization and recruitment of PspA, an ESCRT-III homolog. Under normal conditions, this system transiently assembles at the membrane, stabilizing it and allowing proper motility, secretion, and biofilm formation. Loss of SamI ({Delta}samI/{Delta}ydjI) leads to unbalanced SamG-SamH activity leading to a constitutive stress signalling, and global transcriptional changes reminiscent of starvation situations. This, in turn, blocks secretion of the matrix protein BslA, preventing biofilm formation, and reducing motility. Deletion of samH in combination with {Delta}samI restores biofilm formation, while {Delta}pspA mutants form biofilms normally, indicating that PspA is dispensable for the developmental phenotype. Our findings reveal that beside membrane integrity SamGHI coordinates transcriptional homeostasis and multicellular development through formation of a membrane integral stress sensor complex.