BACTERIOLOGY An Uninhibited Protease

Regulated intramembrane proteolysis (RIP) provides a mechanism whereby proteolytic cleavage of a transmembrane protein can enable the release of a signaling domain. Typically, the regulatory step involves preliminary processing of the substrate, enabling cleavage by the protease. In contrast, processing of the inactive membrane-associated precursor of ^K, a Bacillus subtilis transcription factor involved in sporulation, depends on the release from inhibition of its protease, SpoIVFB (B). B is maintained inactive in a membrane complex, together with SpoIVFA (A) and BofA, until activated by the SpoIVB signaling protein (IVB), a serine protease. Campo and Rudner used an in vitro assay to show that the extracellular domain of A (accessible to IVB in vivo) was cleaved by IVB into three products. IVB also cleaved full-length A solubilized from membrane fractions and green fluorescent protein (GFP)-labeled A complexed with B and BofA in membrane vesicles. After determining the cleavage sites by N-terminal peptide sequencing and by reverse phase high-performance liquid chromatography (HPLC) followed by mass spectrometry, the authors created an uncleavable mutant. Although processing requires IVB, the uncleavable mutant delayed pro-^K processing by about two hours when introduced into B. subtilis but did not block it. However, when introduced into cells lacking a second serine protease, CtpB, the mutant abolished pro-^K processing. CtpB, itself a target of IVB, cleaved the A extracellular domain; furthermore, a mutant resistant to cleavage by both IVB and CtpB blocked pro-^K processing. Thus, the authors propose that IVB regulates ^K activation through a branched proteolytic pathway in which IVB both acts directly on A and activates CtpB to do the same.

N. Campo, D. Z. Rudner, A branched pathway governing the activation of a developmental transcription factor by regulated intramembrane proteolysis. Mol. Cell 23, 25-35 (2006). [PubMed]