Accumulation of Golgi-specific mannosyltransferases in<i>Candida albicans</i>cells grown in the presence of brefeldin A

Abstract

The fungal metabolite brefeldin A (BFA) is known for its ability to block the secretory process in eukaryotic cells by interfering in the endoplasmic reticulum (ER) to Golgi membrane traffic, causing the disassembly 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 whether the incomplete glycosylation of mannoproteins could be due to repression of the synthesis of Golgi-located terminal mannosyltransferases and whether the underglycosylated mannoproteins can be incorporated into the cell walls in Candida albicans. However, we found that the microsomal membranes isolated from the yeast cells grown in the presence of 14 micrograms.mL-1 of BFA had on average three times higher overall specific activity of mannan synthase than membranes from control cells. The increase in specific activity of mannan synthase was mainly due to accumulation of Golgi-specific mannosyltransferases responsible for 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-14C]mannose by the membranes from cells grown in the presence of BFA had longer O-glycosidically linked oligosaccharides and longer side-chains in the N-glycosidically linked polymannose part of the molecule than mannans synthesized by membranes from the control cells. Contrary to results obtained in vitro, the structural features of cell wall mannans isolated from intact BFA-grown and from control cells were almost indistinguishable.