Fig. 1: Steps Involved in Pro-σ^K Cleavage by SpoIVFB.

A Sliced-view diagram of a sporulating B. subtilis cell. The mother cell is shown in red and the forespore is shown in gray. B Cartoon representation of the steps involved in relief of SpoIVFB inhibition and initiation of Pro-σ^K processing during B. subtilis sporulation. In the resting state (state 1) SpoIVFB is held inactive in a complex with BofA and SpoIVFA. The proteases SpoIVB and CtpB are produced in the forespore and secreted into the intermembrane space, where they proteolytically cleave SpoIVFA. Cleavage of SpoIVFA leads to relief of SpoIVFB inhibition, allowing the pro-sequence of Pro-σ^K to engage the active site of SpoIVFB (state 2). The proteolytic activity of SpoIVFB cleaves the pro-sequence of Pro-σ^K (state 3), allowing mature σ^K to diffuse from the vicinity of the outer forespore membrane and initiate further downstream gene transcription in the mother cell cytoplasm and progression of the sporulation process.