In vitro studies on the subcellular location of glucosidase I and glucosidase II in dog pancreas

Abstract

When programmed with yeast prepro-α-factor mRNA, the heterologous reticulocyte/dog pancreas translation system synthesizes two pheromone related polypeptides, a cytosolically located primary translation product (pp-α-F_cyt, 21 kDa) and a membrane-specific and multiply glycosylated e-factor precursor (pp-α-F₃, 27.5 kDa). Glycosylation of the membrane specific pp-α-F₃ species is competitively inhibited by synthetic peptides containing the consensus sequence Asn-Xaa-Thr as indicated by a shift of its molecular mass from 27.5 kDa to about 19.5 kDa (pp-α-F₀) , whereas the primary translation product pp-α-Fcyt is not affected. Likewise, only the glycosylated pp-α-F₃ structure is digested by Endo H yielding a polypeptide with a molecular mass between PP-α-F₀ and pp-α-Fcyt. These observations strongly suggest that the primary translation product is proteolytically processed during/on its translocation into the lumen of the microsomal vesicles. We believe that this proteolytic processing is due to the cleavage of a signal sequence from the pp-α-Fcyt species, although this interpretation contradicts previous data from other groups. The distinct effect exerted by various glycosidase inhibitors (e.g. 1-deoxynojirimycin, N-methyl-dNM, 1-deoxymannojirimycin) on the electrophoretic mobility of the pp-α-F₃ polypeptide indicates that its oligosaccharide chains are processed to presumbly Man₉-GlcNAc₂ structures under thein vitro conditions of translation. This oligosaccharide processing is most likely to involve the action of glucosidase I and glucosidase II as follows from the specificity of the glycosidase inhibitors applied and the differences of the molecular mass observed in their presence. In addition, several arguments suggest that both trimming enzymes are located in the lumen of the microsomal vesicles derived from endoplasmic reticulum membranes.