Animal cells are protected from their surrounding environment by a lipid membrane. However, the membrane must allow external materials, such as nutrients and chemical messengers, to get into the cell. One way this is achieved is through a process called endocytosis, where a small part of the cell membrane folds inward into a pit that pinches off, forming a sack that is free to move through the cell. The protein dynamin is crucial for this process. Our studies and others have shown that dynamin wraps around and constricts the necks of the membrane pits, which helps seal the free-moving sacks.