Direct SpoIIQ-SpoIIIAH interaction is dispensable for sporulation in Bacillus subtilis

Bacillus subtilis sporulation involves a fascinating phagocytic process in which the mother cell engulfs the forespore, internalizing the latter as a cell-within-a-cell. Peptidoglycan remodelling machinery, along with the SpoIIIAA-AH:SpoIIQ complex, are crucial to this process. The forespore protein SpoIIQ and the mother cell protein SpoIIIAH, which localize to opposite sides of the sporulation septum, are indispensable for sporulation. These proteins interact through their extracytoplasmic domains across the intermembrane space and are proposed to contribute to an intercellular zipper and/or a channel connecting the forespore and the mother cell. Here, we show using (1) site-directed mutagenesis of SpoIIQ, (2) in vivo and in vitro interaction and localization studies, and (3) σ^G activation and sporulation assays that spores are formed efficiently from cells in which direct interaction between SpoIIIAH and SpoIIQ (H-Q) is disrupted. We propose that the H-Q interaction is dispensable for sporulation and that the essential function of SpoIIQ is in recruitment of other components to the septum/engulfment complex such as SpoIIE, GerM and/or the other SpoIIIA proteins.