Man9‐Mannosidase from Human Kidney is Expressed in COS Cells as a Golgi‐Resident Type II Transmembrane N‐Glycoprotein

Abstract

Man9‐mannosidase, an α1,2–specific exo‐enzyme involved in N‐linked oligosaccharide processing, has been cloned recently from a human kidney cDNA library [Bause, E., Bieberich, E., Rolfs, A., Volker, C. & Schmidt, B. (1993) Eur. J. Biochem. 217, 533–540], Transient expression in COS 1 cells of the enzyme resulted in a more than 20–fold increase of a catalytic activity cleaving specifically α1,2–mannosidic linkages in [14C]Man9‐GlcNAc2 or [14C]Man5‐GlcNAc2. Man9‐mannosidase is expressed as a N‐glycoprotein with a molecular mass of 73 kDa. Its enzymic activity is metal ion dependent and inhibited strongly by 1–deoxymannojirimycin (50% at 100 μM). Proteolytic studies with the membrane‐associated form of Man9‐mannosidase support the view that the enzyme is a type II transmembrane protein as predicted from its cDNA sequence. Several lines of evidence suggest that Man9‐mannosidase, as expressed, is N‐glycosylated at one of three potential Asn‐Xaa‐Thr/Ser/Cys acceptor sites. Approximately 50% of the N‐linked oligosaccharide chains are removed by endoglycosidase H treatment, whereas complete deglycosylation of the enzyme is observed, when transfected cells were cultured in the presence of the Golgi mannosidase II inhibitor swainsonine, indicating that the sugar moiety of Man9‐mannosidase is processed partially by Golgi‐resident enzymes. This observation is consistent with the results of indirect immunofluorescence studies, pointing to a localization of the Man9‐mannosidase predominantly in the juxtanuclear Golgi region. This localization clearly differs from that of pig liver Man9‐mannosidase which appears to be located in the endoplasmic reticulum and transient vesicles. Copyright © 1995, Wiley Blackwell. All rights reserved