A Branched Pathway Governing the Activation of a Developmental Transcription Factor by Regulated Intramembrane Proteolysis

SummaryThe proteolytic activation of the membrane-associated transcription factor pro-σK is controlled by a signal transduction pathway during sporulation in the bacterium Bacillus subtilis. The pro-σK processing enzyme SpoIVFB, a membrane-embedded metalloprotease, is held inactive by two other integral-membrane proteins, SpoIVFA and BofA. We demonstrate that the signaling protease SpoIVB (IVB) triggers pro-σK processing by cleaving the extracellular domain of the SpoIVFA regulator at multiple sites. In vitro, these cleavages do not disrupt the interactions between SpoIVFA, SpoIVFB, and BofA, suggesting that IVB-dependent activation of the processing enzyme results from a conformational change in this complex. Our data further suggest that when IVB is unable to cleave SpoIVFA, it can still activate pro-σK processing through a second protease, CtpB. Finally, we demonstrate that CtpB, like IVB, triggers pro-σK processing by cleaving SpoIVFA. We propose that IVB regulates intramembrane proteolysis through two proteolytic pathways, both of which converge on the same regulator.