Figure 6. Activation of σ^K during B. subtilis sporulation.

(A) Upon completion of engulfment, the forespore is surrounded by two membranes. Grey box indicates region expanded in (B).

(B) Expanded view depicting a series of proteolytic cleavages (see text for references). First, σ^G in the forespore causes expression of serine proteases SpoIVB and CtpB (also expressed under σ^E control in the mother cell), which are translocated into the intermembrane space, where they cleave the C-terminal domain of SpoIVFA to initiate its degradation (1). SpoIVFA was in a complex with BofA and SpoIVFB in the outer membrane surrounding the forespore after these proteins were expressed in the mother cell under σ^E control. In a second step, CtpB and one or more other proteases (not shown) cleave BofA to initiate its degradation (2). Finally, SpoIVFB cleaves Pro-σ^K, releasing σ^K into the mother cell (3). The pro-sequence of Pro-σ^K is depicted to loop into the membrane based on findings that Pro-σ^K associates peripherally with membranes in B. subtilis (Zhang et al., 1998) or when expressed in E. coli (Zhou et al., 2013).