YND1, a homologue of GDA1, encodes membrane-bound apyrase required for Golgi N- and O-glycosylation in Saccharomyces cerevisiae

The gene for the open reading frame YER005w that is homologous to yeast Gol gi GDPase encoded by the GDA1 gene was cloned and named YND1. It encodes a 630-amino acid protein that contains a single transmembrane region near the carboxyl terminus. The overexpression of the YND1 gene in the gda1 null mu tant caused a significant increase in microsomal membrane-bound nucleoside phosphatase activity with a luminal orientation,The activity was equally hi gh toward ADP/ ATP, GDP/GTP, and UDP/UTP and similar to 50% less toward CDP /CTP and thiamine. pyrophosphate, but there was no activity toward GMP, ind icating that the Ynd1 protein belongs to the apyrase family, This substrate specificity is different from that of yeast GDPase, but similar to that of human Golgi UDPase, The Delta ynd1 mutant cells were defective in O- and N -linked glycosylation in the Golgi compartments. The overexpression of the YND1 gene complemented some glycosylation defects in Delta gda1 disruptants , suggesting a partially redundant function of yeast apyrase and GDPase. Fr om these results and the phenotype of the Delta ynd1 Delta gda1 double dele tion showing a synthetic effect, we conclude that yeast apyrase is required for Golgi glycosylation and cell wall integrity, providing the first direc t evidence for the in vivo function of intracellular apyrase in eukaryotic cells.