Serine proteases from two cell types target different components of a complex that governs regulated intramembrane proteolysis of pro-sigmaK during Bacillus subtilis development

Upon starvation Bacillus subtilis undergoes a developmental process involving creation of two cell types, the mother cell and forespore. A signal in the form of a serine protease, SpoIVB, is secreted from the forespore and leads to regulated intramembrane proteolysis (RIP) of pro-sigmaK, releasing active sigmaK into the mother cell. RIP of pro-sigmaK is carried out by a membrane-embedded metalloprotease, SpoIVFB, which is inactive when bound by BofA and SpoIVFA. We have investigated the mechanism by which this complex is activated. By expressing components of the signalling pathway in Escherichia coli, we reconstructed complete inhibition of pro-sigmaK RIP by BofA and SpoIVFA, and found that SpoIVB serine protease activity could partially restore RIP, apparently by targeting SpoIVFA. Pulse-chase experiments demonstrated that SpoIVFA synthesized early during B. subtilis sporulation is lost in a SpoIVB-dependent fashion, coincident with the onset of pro-sigmaK RIP, supporting the idea that SpoIVB targets SpoIVFA to trigger RIP of pro-sigmaK. Loss of BofA depended not only on SpoIVB, but also on CtpB, a serine protease secreted from the mother cell. CtpB appeared to cleave BofA near its C-terminus upon coexpression in E. coli, and purified CtpB degraded BofA. We propose that RIP of pro-sigmaK involves a three-step proteolytic cascade in which SpoIVB first cleaves SpoIVFA, CtpB then cleaves BofA and finally SpoIVFB cleaves pro-sigmaK.