The phosphoinositide binding Phox Homology domain of Sorting Nexin 27
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. SNX27 contains an N-terminal PDZ domain followed by a PX domain and a Ras-Associated (RA) domain. It binds G protein-gated potassium (Kir3) channels, which play a role in neuronal excitability control, through its PDZ domain. SNX27 downregulates Kir3 channels by promoting their movement in the endosome, reducing surface expression and increasing degradation. SNX27 also associates with 5-hydroxytryptamine type 4 receptor (5-HT4R), cytohesin associated scaffolding protein (CASP), and diacylglycerol kinase zeta, and may play a role in their intracellular trafficking and endocytic recycling. The SNX27 PX domain preferentially binds to phosphatidylinositol-3-phosphate (PI3P) and is important for targeting to the early endosome.
Feature 1: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.