THE SACCHAROMYCES-CEREVISIAE PROCESSING ALPHA-1,2-MANNOSIDASE IS LOCALIZED IN THE ENDOPLASMIC-RETICULUM, INDEPENDENTLY OF KNOWN RETRIEVAL MOTIFS

The yeast-specific alpha 1,2-mannosidase, Mns1p, converts Man(9)GlcNAc (2) to a single isomer of Man(8)GlcNAc(2) during N-linked oligosacchar ide processing in Saccharomyces cerevisiae, Mns1p is a 68 kDa type II integral membrane glycoprotein with a very short amino terminal cytopl asmic fail of only two amino acids and a large carboxy-terminal cataly tic region that is homologous to class 1 alpha 1,2-mannosidases from m ammalian and other species. We have used immunofluorescence and immuno electron microscopy to demonstrate that Mns1p is localized in the endo plasmic reticulum in Saccharomyces cerevisiae. As Mns1p contains none of the known endoplasmic reticulum retrieval motifs (HDEL, KK or RR), these results suggest that Mns1p is localized in the endoplasmic retic ulum by a different retention mechanism.