SNX18 is an SNX9 paralog that acts as a membrane tubulator in AP-1-positive endosomal trafficking

SNX9, SNX18 and SNX30 constitute a separate subfamily of PX-BAR-containing sorting nexin (SNX) proteins. We show here that most tissues express all three paralogs, and immunoprecipitation and immunofluorescence experiments demonstrated that the SNX9-family proteins act as individual entities in cells. Their SH3 domains displayed a high selectivity for dynamin 2, and the PX-BAR units had the capacity to tubulate membranes when expressed in HeLa cells. As previously described for the PX-BAR domain of SNX9 (SNX9-PX-BAR), purified SNX18-PX-BAR caused liposome tubulation in vitro and had a binding preference for PtdIns(4,5)P₂. However, contrary to SNX9, which primarily acts in clathrin-mediated endocytosis at the plasma membrane, endogenous SNX18 localized to AP-1- and PACS1-positive endosomal structures, which were devoid of clathrin and resistant to Brefeldin A. Moreover, a -adaptin recognition motif was defined in a low-complexity region of SNX18, and a complex of endogenous SNX18 and AP-1 could be immunoprecipitated after Brefeldin A treatment. Overexpression of SNX18 sequestered AP-1 from peripheral endosomes and resulted in the formation of short SNX18-decorated tubes with distinct dynamin puncta. The results indicate that SNX9-family members make up discrete membrane-scission units together with dynamin, and suggest that SNX18 mediates budding of carriers for AP-1-positive endosomal trafficking.