ACCUMULATION OF GOLGI-SPECIFIC MANNOSYLTRANSFERASES IN CANDIDA-ALBICANS CELLS GROWN IN THE PRESENCE OF BREFELDIN-A

The fungal metabolite brefeldin A (BFA) is known for its ability to bl ock the secretory process in eukaryotic cells by interfering in the en doplasmic reticulum (ER) to Golgi membrane traffic, causing the disass embly of Golgi apparatus and redistribution of Golgi enzymes into the ER. In sensitive yeasts, underglycosylated forms of secretory proteins accumulate in the cytoplasm in the presence of BFA. We investigated w hether the incomplete glycosylation of mannoproteins could be due to r epression of the synthesis of Golgi-located terminal mannosyltransfera ses and whether the underglycosylated mannoproteins can be incorporate d into the cell walls in Candida albicans. However, we found that the microsomal membranes isolated from the yeast cells grown in the presen ce of 14 mu g . mL(-1) of BFA had on average three times higher overal l specific activity of mannan synthase than membranes from control cel ls. The increase in specific activity of mannan synthase was mainly du e to accumulation of Golgi-specific mannosyltransferases responsible f or elongation of the O-glycosidically linked mannooligosaccharides and for the synthesis of the N-glycosidically linked mannan outer chain. As a consequence, the mannans synthesized in vitro from GDP-[U-C-14]ma nnose by the membranes from cells grown in the presence of BFA had lon ger O-glycosidically linked oligosaccharides and longer side-chains in the N-glycosidically linked polymannose part of the molecule than man nans synthesized by membranes from the control cells. Contrary to resu lts obtained in vitro, the structural features of cell wall mannans is olated from intact BFA-grown and from control cells were almost indist inguishable.