Ewan MacDonald, Sylvie Urbé, Michael J. Clague

The endosomal deubiquitylase USP8 has profound effects on endosomal morphology and organisation. Previous reports have proposed both positive (EGFR, MET) and negative roles in the down-regulation of receptors (Frizzled, Smoothened). Here we report an additional influence of USP8 on the retromer-dependent shuttling of ci-M6PR between the sorting endosome and biosynthetic pathway. Depletion of USP8 leads to a steady state redistribution of ci-M6PR from the Trans-Golgi Network (TGN) to endosomal compartments. Consequently we observe a defect in sorting of lysosomal enzymes, evidenced by increased levels of unprocessed Cathepsin D, which is secreted into the medium. The normal distribution of receptor can be restored by expression of siRNA-resistant USP8 but not by a catalytically inactive mutant or a truncated form, lacking a MIT domain required for endosomal localisation. We suggest that effects of USP8 depletion may reflect the loss of ESCRT-0 components which associate with retromer components Vps35 and SNX1, whilst failure to efficiently deliver lysosomal enzymes may also contribute to the observed block in receptor tyrosine kinase degradation.