The Glyco-enzyme adaptor GOLPH3 Links Intra-Golgi Transport Dynamics to Glycosylation Patterns and Cell Proliferation

Abstract

Glycans are ubiquitous sugar polymers with major biological functions that are assembled by glyco-enzymes onto cargo molecules during their transport through the Golgi complex. How the Golgi determines glycan assembly is poorly understood. By relying on the Golgi cisternal maturation model and using the glyco-enzyme adaptor and oncoprotein GOLPH3 as a molecular tool, we define the first example of how the Golgi controls glycosylation and associated cell functions. GOLPH3, acting as a component of the cisternal maturation mechanism, selectively binds and recycles a subset of glyco-enzymes of the glycosphingolipid synthetic pathway, hinders their escape to the lysosomes and hence increases their levels through a novel lysosomal degradation-regulated mechanism. This enhances the production of specific growth-inducing glycosphingolipids and reprograms the glycosphingolipid pathway to potentiate mitogenic signaling and cell proliferation. These findings unravel unforeseen organizing principles of Golgi-dependent glycosylation and delineate a paradigm for glycan assembly by the Golgi transport mechanisms. Moreover, they indicate a new role of cisternal maturation as a regulator of glycosylation, and outline a novel mechanism of action for GOLPH3-induced proliferation.

Regulated compartmentalization of enzymes in Golgi by GRASP55 controls cellular glycosphingolipid profile and function

“…60µM (23). There was no significant interaction of GRASP domain of GRASP55 with GCS tail deleted of Φ-X-Φ-COOH motif or B4GALT1 tail (Fig.4F).…”

“…While we find that GRASP55 and LCS interact, there is no convincing evidence for their direct interaction so mutating key residues to decompartmentalize the protein similar to what was achieved for GCS is not possible. While studying the molecular basis of LCS localization it was found that the 14 amino acid cytosolic tail of LCS contains the information needed to localize the protein in the Golgi (23). So, we performed alanine mutagenesis of most of the cytosolic tail of LCS (LCS9A) except for the membrane proximal 5 amino acid region essential for interaction with GOLPH3 and the subsequent retention of LCS in Golgi.…”

“…According to the cisternal progression/maturation model, the anterograde flux is mediated by cisternal progression and the retrograde flux by COPI mediated processes (16,17). The mechanistic basis of retrograde transport is relatively well established with recent studies identifying specificity encoders in this transport process (19,23,27,28). The cisternal progression, on the other hand, is considered a bulk-flow/passive process with the retention of proteins in the progressing cisterna secondary to their inefficient interaction with the COPI machinery.…”

“…The retention of Golgi glycosylation enzymes in the face of this continuous forward flux is mediated by their retrograde transport that acts as counterbalance for the forward transport. The retrograde transport is promoted by coat protein complex I (COPI) machinery (18)(19)(20)(21)(22) and is assisted in this process by adaptor molecules like GOLPH3 (23)(24)(25), Conserved oligomeric complex (COG) proteins and Golgi matrix proteins especially Golgins (26)(27)(28). However, the specific molecular mechanisms and processes by which the same retrograde transport pathway promotes localization of enzymes to distinct cisternae remain unknown.…”

Alterations of Golgi Structural Proteins and Glycosylation Defects in Cancer

“…Other than the morphological effect on the Golgi, recent studies provided evidence that overexpression of GOLPH3 exerts its tumor-promoting activities via enhancing the production of specific growth-inducing glycosphingolipids (GSL). Specifically, GOLPH3 functions as an adaptor between a selectively set of Golgi glycosylation enzymes, especially the GSL biosynthetic pathway enzymes, and COPI coatomer (Eckert et al, 2014;Rizzo et al, 2019). The adaptor role of GOLPH3 mediates the incorporation of GOLPH3 clients into the COPI recycling vesicles, but hinders the clients trafficking to the lysosomes and thus increases the protein levels of glycosylation enzymes (Rizzo et al, 2019).…”