An SPFH Protein Couples Membrane Stress to Differentiation in Bacillus subtilis

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 (ΔsamI/ΔydjI) 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.