Figure 1

Ratchet model for the zipper mechanism in phagocytosis. (A) Schematic of the zipper mechanism. Cell-membrane receptors (green dots) and ligands on the particle surface (blue dots) are sequentially engaged in bond formation, resulting in progression of engulfment with time (from left to right). (B) Schematic of our ratchet model. (left) A random membrane fluctuation (blue) far from the particle is unable to trigger ligand-receptor binding and signaling. Therefore it is not supported by remodeling of the actin cortex (straight red line), and the membrane may move backwards at a later time. (right) A membrane fluctuation near the particle leads to ligand-receptor binding, resulting in signaling and actin polymerization (red diamonds). Consequently, the actin cortex is deformed to support the membrane fluctuation, which makes the membrane move irreversibly for zipper progression. Energetic costs and gains of membrane fluctuations used in model are shown as well.