Differential Lipid Binding Properties and Physiological Functions of Sorting Nexin Family Proteins

Sorting nexin proteins (SNXs) are a group of cellular proteins that contain the PX domain. Although they have been implicated in membrane trafficking, physiological functions of most SNXs still remain unknown. Many PX domains in SNXs preferably interact with phosphatidylinositol-3-phosphate (PtdIns(3)P) that is enriched in early endosomes during membrane trafficking, but PX domains from other families of proteins have been reported to bind other phosphoinositides as well. To explore the possibility that SNX PX domains may also bind other phosphoinositides and thereby mediate different cellular functions for SNXs, we investigated the lipid binding properties and cellular localization behaviors of sixteen different SNXs by surface plasmon resonance (SPR) analysis and fluorescence microscopy, respectively. Results showed that the PX domains of SNXs and full-length SNXs have highly variable lipid binding and cellular membrane binding properties. The PX domains in SNX3, SNX12, SNX13, SNX29 and SNX32 showed the PtdIns(3)P selectivity but did not have high enough membrane affinity to drive endosomal localization of proteins in mammalian cells. The PX domains of SNX8, SNX16 and SNX24 bound to PtdIns(3)P selectively and tightly, providing the driving force for endosomal localization of their host proteins and allowing them to function in endosomal sorting and cargo trafficking. The PX domains of SNX15 and SNX27 caused vesicle deformation when overexpressed in cells, suggesting a potential role in endosome homeostasis. Interestingly, SNX15-PX can bind phosphatidylinositol-5-phosphate (PtdIns(5)P) and it implies a potential function of SNX15 in the nucleus. Also, SNX4-PX also bound phosphatidylinositol-3,4-bisphosphate (PtdIns(3,4)P2), phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P2) and phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) beside PtdIns(3)P, indicating that SNX4 might be involved in cellular process on plasma membrane, early endosomes and late endosomes. Among all the SNXs tested, we selected SNX21 for detailed functional study because of its unique subcellular localization pattern. Similar to many other SNXs, SNX21 selectively bound PtdIns(3)P and was localized to endosomes when expressed in mammalian cells. However, it underwent the truncation of the N-terminal region, which uncovered the nucleus localization signal and promoted translocation to the nucleus. Biochemical assays showed that SNX21 had unprecedented metal-dependent nuclease activity, with its catalytic activity comparable to prototypical nuclease, DNase I. Furthermore, our cell studies showed that SNX21 caused DNA fragmentation and apoptosis when overexpressed in mammalian cells. This new discovery suggests that SNX21 is a novel dual specificity protein that can interact with both membrane lipids and nucleic acids and that it may serve as a new class of endonuclease that plays a role in apoptosis.