Organizational Interplay of Golgi N-Glycosyltransferases Involves Organelle Microenvironment-Dependent Transitions between Enzyme Homo- and Heteromers

Abstract

Background: Glycans are synthesized in the Golgi by glycosyltransferase complexes, but their functional organization is unclear. Results: Organizational interplay and trafficking of glycosyltransferase complexes involves dynamic transitions between enzyme homo-and heteromers. Conclusion: Organellar microenvironmental factors play a crucial role in the functional organization of the Golgi glycosylation pathways. Significance: Organizational interplay between glycosyltransferase complexes provides new insights into glycosylation and associated diseases.

Glycosyltransferase complexes in eukaryotes: long-known, prevalent but still unrecognized

“…The remaining CESAs (CESA2, CESA5, CESA9, and CESA10) are likely involved in tissue-specific processes and are partially redundant with CESA6 [85]. Intriguingly, these enzymes were also shown to homodimerize prior to formation of larger CESA oligomers [84], similar to N-glycosyltransferases found in mammalian cells [86]. Very recently, a split-ubiquitin membrane yeast twohybrid system demonstrated interactions between the four primary CESAs (CESA1, CESA2, CESA3, CESA6) and three secondary CESAs (CESA4, CESA7, CESA8) but also between the primary CESAs and secondary CESAs in a limited fashion.…”

“…Fourthly, the observed pH sensitivity of the enzyme heteromers, but not of homomers [41], is important to notice, and reflects their formation in different cellular compartments. The latter were shown to form in the ER whereas the former are now known to assemble in the acidic Golgi environment [86]. This result emphasizes that homomers and heteromers are not competing enzyme species, but rather represent inter-dependent membrane constituents that undergo constant organelle micro-environment-dependent transitions between two physical states during their suggested recycling between the early secretory compartments (Fig.…”

“…3). Such transitions indeed take place between the ER and the Golgi [86], and provide a simple means to localize the most active enzyme species (heteromers) in the Golgi where these enzymes are known to operate, and thereby can increase the glycosylation potential of the Golgi. The absence of enzyme heteromers in acidification and glycosylation -defective cancer cells is in accordance with this view.…”

“…However, the now generally accepted ''cisternal maturation'' model is not in accord with this oligomerizationmediated Golgi retention hypothesis, as it assumes that the Golgi enzymes continuously recycle between Golgi cisternae and the ER. In fact, it has been shown by using the FRAP (fluorescence recovery after photobleaching) approach that glycosyltransferases in general [116] as well as their homomers and heteromers [86] remain as mobile Golgi membrane constituents that recycle between the first two secretory compartments. Thus, even though some enzymes are mislocalized after experimental Golgi pH increase [99], it is currently not clear whether complex formation itself is a determinant for Golgi retention or retrieval.…”

Brefeldin A promotes the appearance of oligosaccharyl phosphates derived from Glc3Man9GlcNAc2-PP-dolichol within the endomembrane system of HepG2 cells

“…There is a progressive acidification of organelles of the endomembrane system from pH 7.2 in the ER to pH 6.0 in the trans Golgi network (56), and studies show that GA-situated glycosyltransferases form biologically important homo-and heterodimers at lower pH (57,58). Therefore enzymes normally resident in distal GA cisternae are perhaps less active in the BFA compartment, whose pH is likely to be similar to that of the ER (59).…”