ENDOPLASMIC-RETICULUM SUBCOMPARTMENTS IN A PLANT-PARASITIC FUNGUS ANDIN BAKERS-YEAST - DIFFERENTIAL DISTRIBUTION OF LUMENAL PROTEINS

His-Asp-Glu-Leu (HDEL)-bearing proteins were quantified in different e ndoplasmic reticulum (ER) subcompartments of Saccharomyces cerevisiae and the plant parasite Uromyces viciae-fabae by immune-electron micros copy (immuno-EM). In both fungi, the immunogold labeling of these prot eins within the ER was three times greater than within the nuclear env elope. In U. viciae-fabae, the ER in germinating uredospores differed from the ER in fungal structures produced within the plant, e.g., haus toria. In haustoria, the cisternal ER differentiated large tubular-ves icular complexes (TVC). TVC contained higher levels of HDEL-bearing pr oteins than ordinary ER cisternae. ELISA readings also indicated an in creased concentration of these proteins in isolated haustoria compared to germinating uredospores. In S. cerevisiae, the ER was differentiat ed into cortical and internal regions. Immuno-EM revealed that labelin g of the binding protein (BiP) was lower in the ER of the cell cortex. Heat shock increased BiP signals, but the relative distribution withi n the ER did not change. Our results suggest that ER subcompartments c an be differentiated by immunogold labeling of proteins with a retenti on signal. In special cases, such as in the parasitic phase of rust fu ngi, these proteins accumulate to higher levels in ER subcompartments, probably as a response to plant-induced stress. (C) 1995 Academic Pre ss, Inc.