Solution Structure of SpoIVB Reveals Mechanism of PDZ Domain-Regulated Protease Activity

Intramembrane proteases hydrolyze peptide bonds within the cell membrane as the decision-making step of various signaling pathways. Sporulation factor IV B protease (SpoIVB) and C-terminal processing proteases B (CtpB) play central roles in cellular differentiation via regulated intramembrane proteolysis (RIP) process which activates pro-σ^K processing at the σ^K checkpoint during spore formation. SpoIVB joins CtpB in belonging to the widespread family of PDZ-proteases, but much remains unclear about the molecular mechanisms and structure of SpoIVB. In this study, we expressed inactive SpoIVB (SpoIVB^S378A) fused with maltose binding protein (MBP)-tag and obtained the solution structure of SpoIVB^S378A from its small angle X-ray scattering (SAXS) data. The fusion protein is more soluble, stable, and yields higher expression compared to SpoIVB without the tag. MBP-tag not only facilitates modeling of the structure in the SAXS envelope but also evaluates reliability of the model. The solution structure of SpoIVB^S378A fits closely with the experimental scattering data (χ²= 1.76). Comparing the conformations of PDZ-proteases indicates that SpoIVB adopts a PDZ-protease pattern similar to the high temperature requirement A proteases (HtrAs) rather than CtpB. We not only propose that SpoIVB uses a more direct and simple way to cleave the substrates than that of CtpB, but also that they work together as signal amplifiers to activate downstream proteins in the RIP pathway.