Decision letter: The membrane-associated proteins FCHo and SGIP are allosteric activators of the AP2 clathrin adaptor complex

Abstract

All cells are enveloped by a plasma membrane. To interact with the outside world, cells constantly recycle the molecules found in, or on, this barrier. This is accomplished by drawing in small patches of the membrane containing these ‘cargo’ molecules via a process called endocytosis. The predominant method of endocytosis involves coating the tiny membrane pouches with a scaffold-like structure made of clathrin molecules. However, clathrin requires a set of four proteins (known as the adaptor protein-2 complex) to connect the membrane and cargo to the clathrin cage. Previous studies have suggested that the adaptor protein-2 complex may exist in at least two forms: one in which the binding sites for membrane and cargo are hidden, and another where these sites are exposed. These structures were proposed to represent inactive (closed) and active (open) forms of the complex, respectively. It has been unclear whether reorganization of the adaptor complex is a necessary step in endocytosis or how it might be stimulated. Now Hollopeter et al. show that worms that lack a membrane-associated protein called FCHo are unable to cluster the adaptor protein-2 complex on their cell membranes, and their cells have difficulties taking up cargo. When the FCHo protein was missing, the adaptor protein-2 complex remained in its closed shape, suggesting that the FCHo protein is needed to switch the complex from its closed to its open structure. When Hollopeter et al. looked for worms with genetic changes that can overcome the defects caused by a lack of FCHo, they identified worms with various mutations in the genes for the adaptor protein-2 complex. These mutations altered the proteins in the complex at positions that are predicted to rearrange dramatically when the complex is activated; Hollopeter et al. confirmed that such rearrangements do occur in living worms. Furthermore, Hollopeter et al. found that giving mutant worms, which lacked the fcho gene, a small fragment of the FCHo protein causes the adaptor protein-2 complex to adopt its open structure. Similar fragments from other related membrane-associated proteins had the same effect, and these fragments all ‘cured’ the worms' endocytosis problems. The FCHo fragment directly binds the adaptor complex and Hollopeter et al. propose that FCHo proteins function to activate this complex at the sites where endocytosis occurs.