“Reappraisal of the role of nucleoside diphosphatases on yeast N- and O-glycosylation” .

CECILIA D'ALESSIO; ARMANDO J. PARODI

Ventura, CA, USA

Conferencia; Conferencia Gordon de Investigación en Glicobiología; 2005

It is accepted that glycosyltransferase-generated nucleoside diphosphates are converted to monophosphates in the secretory pathway by nucleoside diphosphatases (NDPases) to provide substrates for antiport transport systems by which entrance of nucleotide sugars from the cytosol into the lumen is coupled to exit of nucleoside monophosphates. Working with Saccharomyces cerevisiae mutants defective in one or both secretory pathway NDPases and/or in both anterograd and retrograde ER-Golgi vesicular traffic we show that although synthesis of large N- and O-glycans, processes that presumably occur in the medial and trans Golgi cisternae, do indeed require the presence of NDPases in the secretory pathway, entrance of both UDP-Glc and GDP-Man into the ER and of the last compound into the cis Golgi cisternae may occur in the absence of the enzymes. Moreover, we show that although nucleotide sugar-dependent N-glycan elongation starts in the Golgi, synthesis of large O-glycans may occur in the ER, but exclusively on misfolded glycoproteins. Further, we show that the absence of the Man 1-P transferase (a GMP generating enzyme) does not prevent entrance of GDP-Man into the ER and cis Golgi lumen in the absence of NDPases. These and other results suggest the existence of additional, as yet unidentified nucleotide sugar transporters and the role of large O-glycans in the ER as signals for driving misfolded glycoproteins to degradation.