Retromer has a selective function in cargo sorting via endosome transport carriers

Retromer is a peripheral membrane protein complex that coordinates multiple vesicular trafficking events within the endolysosomal system. Here, we demonstrate that retromer is required for the maintenance of normal lysosomal morphology and function. The knockout of retromer subunit Vps35 causes an ultrastructural alteration in lysosomal structure and aberrant lysosome function, leading to impaired autophagy. At the whole-cell level, knockout of retromer Vps35 subunit reduces lysosomal proteolytic capacity as a consequence of the improper processing of lysosomal hydrolases, which is dependent on the trafficking of the cation-independent mannose 6-phosphate receptor (CI-M6PR). Incorporation of CI-M6PR into endosome transport carriers via a retromer-dependent process is restricted to those tethered by GCC88 but not golgin-97 or golgin-245. Finally, we show that this retromer-dependent retrograde cargo trafficking pathway requires SNX3, but not other retromer-associated cargo binding proteins, such as SNX27 or SNX-BAR proteins. Therefore, retromer does contribute to the retrograde trafficking of CI-M6PR required for maturation of lysosomal hydrolases and lysosomal function.