Retrograde Transport of Golgi Enzymes by GOLPH3 Across Maturing Cisternae Regulates Glycan Assembly on Sphingolipids and Cell Growth

Abstract We have investigated how glycans, ubiquitous polysaccharides endowed with major biological functions, are assembled by glyco-enzymes on cargo proteins/ lipids during their transport through the Golgi complex. We have manipulated glyco-enzymes retrotransport, a core process in intra-Golgi trafficking, through controlled alterations of the levels of GOLPH3, a coatomer adaptor that drives the retrograde movement of glyco-enzymes and shows oncogenic properties in humans, and examined glycan assembly. We find that: a) GOLPH3 is part of the cisternal maturation mechanism, yet it specifically binds and sorts for recycling a subset of enzymes involved in glycan assembly on sphingolipids, while other enzymes recycle independently of GOLPH3. This indicates that cisternal maturation is an ensemble of enzyme-recycling modules; b) GOLPH3 increases the levels of its client enzymes via a regulated-degradation mechanism based on delaying their escape from the Golgi into the lysosomes; and, c) by changing the levels of key enzymes, GOLPH3 alters the glycan patterns assembled on sphingolipids, to produce more proliferation-inducing glycosphingolipid metabolites. These findings unravel organizing principles of intra-Golgi traffic and of glycan assembly that might have general applicability and broad pathophysiological implications, and outline a novel mechanism for GOLPH3-induced proliferation based on glycosphingolipid biosynthesis.