Figure 1. The Process of Engulfment during B. subtilis Sporulation.

(A) The smaller forespore (FS) and larger mother cell (MC) initially lie side by side. Engulfment commences with septal thinning (i), during which septal peptidoglycan (light gray) is degraded. The mother cell membrane then migrates around the forespore (steps ii–iii), until it meets and fuses to release the forespore into the mother cell cytoplasm (step iv).

(B) Engulfment in intact cells requires three mother cell membrane proteins, SpoIID (Pac-Man), SpoIIM (dotted box), and SpoIIP (shaded lollipop), that localize to the septum and leading edge of the engulfing membrane. SpoIID is a peptidoglycan hydrolase, suggesting that engulfment might be mediated by the processive hydrolysis of the peptidoglycan adjacent to the forespore membrane, which could move the mother cell membrane around the forespore (in direction indicated by the arrow). Figure based on Abanes-De Mello et al. (2002).

(C) The zipper-like interaction between the forespore membrane protein SpoIIQ (Q; gradient) and the mother cell membrane protein SpoIIIAH (AH; gray) localizes SpoIIIAH (Blaylock et al., 2004; Doan et al., 2005), which recruits additional mother cell proteins (Doan et al., 2005; Jiang et al., 2005).

(D) A mechanical ratchet: the stationary pawl engages the ratchet teeth (or cogs), thereby preventing backward rotation of the wheel, allowing movement only in the direction indicated by the arrow. In a Brownian ratchet, random thermal energy is rectified, resulting in unidirectional rotation of the ratchet.