We have identified an intracellular protein, called sorting nexin 9 (SNX9), that play a crucial role for the release of transport vesicles formed at the plasma membrane. Studies of the biochemical properties of recombinant and endogenous SNX9 have brought forward this protein as a recruiter of dynamin to the neck of clathrin-coated vesicles. Members of the dynamin family of large GTPases are involved in membrane fission events that take place during intracellular trafficking of proteins and lipids. Dynamin oligomerization at the base of the bud stimulates the hydrolysis of GTP, which leads to constriction through a conformational change and deformation of the membrane. This activity needs to be restricted to membrane areas that possess high curvature like the vesicular neck.
SNX9 belongs to a subfamily of sorting nexins that in addition to a phosphoinositide-specific PX (phox) domain has a membrane-sculpting BAR (Bin/amphiphysin/Rvs) domain. The domain structure of SNX9 is well adapted to fit into the complex network of interacting proteins that come together to generate a clathrin coated vesicle at the plasma membrane. An amino terminal SH3 (src homology 3) domain binds to the proline-rich region of dynamin. This interaction is strong enough to form an endogenous complex between SNX9 and dynamin already in the cytosol. To link into the vesicle budding network, SNX9 has an unstructured region (LC region) with motifs for binding to the core components AP-2 and clathrin. The PX-BAR unit of SNX9 targets the SNX9-dynamin complex to the plasma membrane. In combination with the interactions with AP-2 and clathrin, the membrane-binding properties of SNX9 are anticipated to precisely target SNX9 to the highly curved neck of the clathrin coated pit.
We have reported the crystal structures of the functional membrane remodeling PX-BAR unit of SNX9 and have shown that it efficiently tubulates lipid membranes in vivo and in vitro. Elucidation of the protein superdomain structure, together with mutational analysis and biochemical and cell biological experiments, have demonstrated how the SNX9 PX and BAR domains work in concert in targeting and tubulation of phosphoinositide-containing membranes. These studies provide insights into the SNX9-induced membrane-modulation mechanism.
SNX9, SNX18, and SNX33 constitute a separate sub-family of PX-BAR-containing sorting nexin proteins. On-going studies aim to understand the specific roles of SNX18 and SNX33. Results have shown that SNX18 is a positive regulator of autophagosome biogenesis. SNX18 interacts with autophagy factors ATG16L1 and LC3, and functions downstream of ATG14 and the class III PtdIns3K complex in autophagosome formation. SNX18 facilitates recruitment of ATG16L1 to perinuclear recycling endosomes, and its overexpression leads to tubulation of ATG16L1- and LC3-positive membranes. We propose that SNX18 promotes LC3 lipidation and tubulation of recycling endosomes to provide membrane for phagophore expansion.