GOLPH3 and GOLPH3L maintain Golgi localization of LYSET and a functional mannose 6-phosphate transport pathway

Glycosylation plays an important role for modifications of lipids and sorting of proteins that is regulated by asymmetric intra-Golgi distribution and SPPL3-mediated cleavage of Golgi enzymes. We found that cells lacking the Golgi N-acetylglucosamine phosphotransferase (GNPT) retention factor LYSET/TMEM251 display a SPPL3-dependent hypersecretion of the galactosyltransferase B4GALT5 which is caused by the loss of its lysosomal mannose 6-phosphate (M6P) targeting signal. Similarly, loss of the COPI adaptors GOLPH3/GOLPH3L destabilized LYSET-GNPT complexes and led to a prominent hypersecretion of B4GALT5 and lysosomal enzymes. Mechanistically, we identified LYSET as a novel, atypical client of GOLPH3/GOLPH3L. GOLPH3/GOLPH3L hence ensure cis-Golgi localization of the LYSET-GNPT complex and maintain Golgi polarity, integrity of the M6P tagging machinery and homeostasis of lysosomes.