Model for spastin mode of action and a proposed HSP cellular pathway. (A) Schematic of the proposed ER–endosome contacts mediated by spastin and IST1. The CHMP1B and IST1 complex implicated in fission coats the endosomal tubule. A spastin hexameric complex is in orange, with the ATPase domains positioned to cut a microtubule, but for clarity the N-terminal tails of a single M87-spastin and M1-spastin molecule are shown. (B) Schematic diagram of key trafficking pathways influenced by spastin and IST1. Spastin and IST1 drive ER-mediated fission of tubules, decorated by sorting nexins, from the early sorting endosome, thus promoting endosome-to-Golgi traffic of ciM6PR and recycling of TfnR. CiM6PR then captures newly synthesized lysosomal enzymes at the Golgi, and the complex is then trafficked back to endosomes, to deliver lysosomal enzymes to the late endosomal/lysosomal degradative compartment. For simplicity, only a single ER tubule is shown. Inset, 3D view of the relationship between endosomal tubules, ER tubules, microtubules, and spastin (orange) and IST1 (purple). (C) In cells lacking spastin, fission of endosomal tubules is inhibited (black rectangles), so ciM6PR and TfnR are not efficiently sorted away from the endosome and instead are trafficked to the degradative compartment. Reduced availability of ciM6PR at the Golgi apparatus results in increased secretion and reduced delivery to the endosomal pathway of lysosomal enzymes. This causes lysosomal morphological and functional abnormalities, including increased lysosomal size and the presence of abnormal arrays of dense membrane. (D) Proposed sites of action of selected HSP proteins involved in membrane traffic, overlaid onto the trafficking pathways shown in B.