During the extracytoplasmic stress response in Escherichia coli, the intramembrane protease RseP cleaves the anti-sigma(E) protein RseA only after the membrane-anchored protease DegS truncates the perk plasmic part of RseA that suppresses the action of RseP. Here we analyzed the three-dimensional structure of the two tandemly arranged PSD-95/DIg/ZO-1 (PDZ) domains (PDZ tandem) present in the periplasmic region of RseP and revealed that the two putative ligand-binding grooves constitute a single pocket-like structure that would lie just above the active center sequestrated within the membrane. Complete removal of the PDZ tandem from RseP led to the intramembrane cleavage of RseA without prior truncation by DegS. Furthermore, mutations expected to destabilize the tertiary structure of the PDZ tandem also caused the deregulation of the sequential cleavage. These observations suggest that the PDZ tandem serves as a size-exclusion filter to accommodate the truncated form of RseA into the active center.