Abstract
Adaptor protein 4 (AP-4) is an ancient membrane trafficking complex, whose function has largely remained elusive. In humans, AP-4 deficiency causes a severe neurological disorder of unknown aetiology. We apply multiple unbiased proteomic methods, including ‘Dynamic Organellar Maps’, to find proteins whose subcellular localisation depends on AP-4. We identify three highly conserved transmembrane cargo proteins, ATG9A, SERINC1 and SERINC3, and two AP-4 accessory proteins, RUSC1 and RUSC2. We demonstrate that AP-4 deficiency causes missorting of ATG9A in diverse cell types, including neuroblastoma and AP-4 patient-derived cells, as well as dysregulation of autophagy. Furthermore, we show that RUSC2 facilitates the microtubule plus-end-directed transport of AP-4-derived, ATG9A-positive vesicles from the TGN to the cell periphery. Since ATG9A has essential functions in neuronal homeostasis, our data not only uncover the ubiquitous function of the AP-4 pathway, but also begin to explain the molecular pathomechanism of AP-4 deficiency.
