PROTEOLYTIC PROCESSING OF A SECRETED GLYCOPROTEIN AND O-GLYCOSYLATIONOF MANNOPROTEINS ARE AFFECTED IN THE N-GLYCOSYLATION MUTANT SACCHAROMYCES-CEREVISIAE LDB1

In a previous work [P.I. Manas, I. Olivero, M. Avalos, L.M. Hernandez, Glycobiology, 7 (1997) 487-497], we described the isolation and chara cterization of the Saccharomyces cerevisiae ldb1 mutant which is affec ted in several steps of the N-glycosylation of mannoproteins probably due to a malfunction of the Golgi apparatus. Here, we found that two f urther functions assigned to the Golgi cisternae are also affected in the mutant: proteolytic processing of a secreted protein and O-glycosy lation. We found that around 70% of the exoglucanase activity that is secreted into the culture medium by ldb1 bears an extra tetrapeptide i n its NH2-terminus due to incomplete proteolytic processing. The O-lin ked oligosaccharides from ldb1 mnn1 were indistinguishable from those synthesized by the parental strain mnn1. However, when the O-oligosacc harides from the wild type and ldb1 were compared, we found a signific ant decrease in the tetrasaccharide in the latter, as well as a concom itant increase in the disaccharide, suggesting a defect in the Kre2p/M nt1p involved in the transfer of the third mannose of these residues. (C) 1998 Elsevier Science B.V.