(A) Brownian ratchet model for protoplast engulfment. During protoplast formation, peptidoglycan hydrolases (scissors) remove the peptidoglycan (hatched arcs) that blocks the engulfing membrane, allowing the Q-AH ratchet to engage and prevent backward membrane movement. The mother cell membrane could move by Brownian motion (double-headed arrows).
(B) Models for engulfment in intact cells. (1) Missing motor model. The DMP proteins (Pac-Man) might act like lysozyme, degrading the peptidoglycan that blocks engulfment. Since SpoIIQ and SpoIIIAH are not essential for engulfment in whole cells, this model requires additional unidentified force-generating proteins (X arrow). (2) Dual ratchet model. The membrane-anchored DMP proteins might act as a burnt-bridge Brownian ratchet, in which the enzymatic activity of SpoIID renders complex diffusion along the peptidoglycan unidirectional, thereby moving the engulfing membrane around the forespore. The Q-AH ratchet could contribute to the directionality and processivity of the DMP ratchet.