Fig. 3.

Current views of the engulfasome in B. subtilis and C. difficile

In B. subtilis, SpoIIM (pink) recruits SpoIIP (purple) which in turn recruits SpoIID (magenta) to the mother cell membrane. This machinery is proposed to help recruit the Q:AH (blue and orange, respectively) to the septum and the leading edges of the membrane during engulfment.

In C. difficile, SpoIIP is transcribed on the forespore side and recent work by Ribis et al. (2018) suggests CdIIP may be proteolytically processed and not necessarily membrane anchored. SpoIIIAH (orange) and SpoIIQ (blue) interact across the intramembrane space holding the mother cell and forespore together. Direct interactions between CdIID and CdIIP with Q:AH suggest a multimeric complex, part of the engulfasome, that is required for engulfment. Identification of other proteins involved and the exact nature of the interactions within the engulfasome need to be investigated. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)