A sporulation membrane protein tethers the pro-σK processing enzyme to its inhibitor and dictates its subcellular localization

The developmental transcription factor sigma(K) is derived from the inactive precursor protein pro-sigma(K) by regulated proteolysis during the process of sporulation in the bacterium Bacillus subtilis. The putative pro-sigma(K) processing enzyme SpoIVFB is a member of a family of membrane-embedded metalloproteases and is held inactive by two other integral membrane proteins, SpoIVFA and BofA. Herein we show that the processing enzyme and its two regulators exist in a multimeric complex that localizes to the membrane surrounding the developing spore (the forespore). We further show that one of the regulators, SpoIVFA, plays a central role in both the formation of this complex and its subcellular localization. Evidence is presented in support of a model in which SpoIVFA acts as a platform for bringing BofA and SpoIVFB together, whereby BofA inhibits pro-sigma(K) processing until a signal has been received from the forespore.