The phosphoinositide binding Phox Homology domain of Sorting Nexin 5
The PX domain is a phosphoinositide (PI) binding module present in many proteins with diverse functions. Sorting nexins (SNXs) make up the largest group among PX domain containing proteins. They are involved in regulating membrane traffic and protein sorting in the endosomal system. The PX domain of SNXs binds PIs and targets the protein to PI-enriched membranes. SNXs differ from each other in PI-binding specificity and affinity, and the presence of other protein-protein interaction domains, which help determine subcellular localization and specific function in the endocytic pathway. SNX5, abundantly expressed in macrophages, regulates macropinocytosis, a process that enables cells to internalize large amounts of external solutes. It may also be a component of the retromer complex, a membrane coat multimeric complex required for endosomal retrieval of lysosomal hydrolase receptors to the Golgi, acting as a mammalian equivalent of yeast Vsp17p. It also binds the Fanconi anaemia complementation group A protein (FANCA). SNX5 harbors a Bin/Amphiphysin/Rvs (BAR) domain, which detects membrane curvature, C-terminal to the PX domain, similar to other sorting nexins including SNX1-2. The PX-BAR structural unit helps determine the specific membrane-targeting of some SNXs. The PX domain of SNX5 binds phosphatidylinositol-3-phosphate (PI3P) and PI(3,4)P2. SNX5 is localized to a subdomain of early endosome and is recruited to the plasma membrane following EGF stimulation and elevation of PI(3,4)P2 levels.
Feature 1:putative phosphoinositide binding site [chemical binding site]
Comment:A majority of PX domain containing proteins binds phosphatidylinositol-3-phosphate (PI3P) at this site. In some cases, other phosphoinositides, such as PI4P or PI(3,4)P2, are the preferred substrates.
Comment:based on the structures of phosphatidylinositol-3-phosphate bound to other members of this superfamily
Comment:Two basic residues are key in binding with phosphoinositides: one forms hydrogen bonds with the 3-phosphate of PI(3)P and another forms hydrogen bonds with the 4-and 5-hydroxyl groups of PI(3)P.