Oligosaccharide trimming Man9-mannosidase is a resident ER protein and exhibits a more restricted and local distribution than glucosidase II.

The subcellular distribution of a recently described neutral trimming mannosidase, the Man9-mannosidase, has been investigated by immunoelectron microscopy in pig hepatocytes. This enzyme processes asparagine-linked N-acetylglucosamine2 mannose9 oligosaccharides by specifically cleaving three of the four alpha 1,2-linked mannose residues and is distinct from the presumptive ER alpha-mannosidase and the two Golgi alpha-mannosidases. Specific polyclonal antibodies for Man9-mannosidase have been prepared, affinity-purified and characterized by immunoblotting. Immunolabeling using these antibodies was observed in the rough (rER) and smooth endoplasmic reticulum (sER) as well as transitional elements of the rough endoplasmic reticulum and smooth membrane profiles close to the Golgi apparatus while the cisternal stack of the Golgi apparatus was unlabeled. This indicates that Man9-mannosidase is an ER trimming mannosidase which acts on glycoproteins after glucosidase II trimming and before their transport to the Golgi apparatus. A comparison with glucosidase II by double immunolabeling showed a more restricted and local distribution of Man9-mannosidase since it was undetectable in the nuclear envelope and although present throughout the rER and sER, large portions of rER cisternae and many sER profiles were unlabeled. This local distribution of Man9-mannosidase might provide a morphological basis for its involvement in selective trimming reactions in the ER. The catabolic pathway for Man9-mannosidase, as for glucosidase II, may involve degradation in autophagic vacuoles.