An investigation of the function of adaptor protein complex 4 (AP-4)
Vesicle trafficking provides the solution to the ‘sorting problem’ – how the eukaryotic cell maintains the distinct identities, and thus functional properties, of its membrane-bound organelles. During vesicle trafficking, proteins are selectively sorted into membrane bound transport intermediates by vesicle adaptors, which include those of the highly conserved adaptor protein (AP) complex family. Each AP complex has a distinct subcellular localisation and functions in the sorting of a specific subset of transmembrane cargo proteins. Adaptor protein complex 4 (AP-4) is one of the more recently identified AP complexes, whose function has largely remained elusive. In humans, AP-4 deficiency causes a severe neurological disorder, suggesting an important role in neuronal development and homeostasis. However, the pathomechanisms that underly the neuronal pathology in AP-4 deficiency are currently unknown. AP-4 is proposed to function in protein sorting at the trans-Golgi network (TGN), so AP-4 deficiency can be thought of as a disease of missorting. The aim of this study was to apply unbiased global proteomic approaches to define the composition of AP-4 vesicles and to identify physiological cargo proteins of the AP-4 pathway. Using ‘Dynamic Organellar Maps’ and comparative analysis of vesicle-enriched fractions from wild-type and AP-4-depleted cells, three ubiquitously expressed transmembrane cargo proteins, ATG9A, SERINC1 and SERINC3, were found to be mislocalised in AP-4-deficient cells. Two novel cytosolic AP-4 accessory proteins, RUSC1 and RUSC2, were also identified. Further proteomic analyses confirmed the interactions between these proteins. AP-4 deficiency was found to cause missorting of ATG9A in diverse cell types, including patient derived cells, as well as dysregulation of autophagy. RUSC2 facilitates the transport of AP-4-derived, ATG9A and SERINC-positive vesicles from the TGN to the cell periphery. These vesicles cluster in close association with autophagosomes, suggesting they are the ‘ATG9 reservoir’ required for autophagosome biogenesis. This study uncovers ATG9A trafficking as a ubiquitous function of the AP-4 pathway. Furthermore, it provides a potential molecular pathomechanism of AP-4 deficiency, through dysregulated spatial control of autophagy.
Membrane trafficking, Proteomics, Autophagy, Molecular mechanisms of disease, Adaptor protein complex, Hereditary spastic paraplegia
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