Cells in the human body constantly receive substances from the outside via a process called endocytosis. One important endocytosis pathway involves the formation of a protrusion within the cell membrane, pointing to the inside of the cell, triggered when a molecule needing to get in reaches the membrane. The protrusion wraps and closes around the molecule, after which it is cut off, leaving the wrapped molecule (a so-called vesicle) inside the cell. A key role in the cutting-off mechanism is played by dynamin, a protein that can locally ‘constrict’ cell membranes, and chop off bits. The precise constriction mechanism is not completely understood, however. Now, by combining experiments and simulations probing the dynamics of dynamin, Alexander Mikhailov from Kanazawa University and colleagues show that the workings of such a ‘nanomuscle’ resembles that of a ratchet motor.