Topological investigations. Study of the trypsin sensitivity of the N-acetylglucosaminyl and mannosyl-transferase activities located in the outer mitochondrial membrane.

The trypsin sensitivity of the mitochondrial N-acetylglucosaminyl and mannosyltransferase activities involved in the N-glycoprotein biosynthesis through dolichol intermediates as well as the N-acetylglucosaminyl-transferase activity involved in direct N-glycosylation were examined in mitochondria and isolated outer mitochondrial membrane preparations. The trypsin action on mitochondrial membrane was checked by measuring the activities of marker enzymes (rotenone-insensitive NADH cytochrome c reductase, adenylate kinase, and monoamine oxidase). Glycosyl-transferase activities of both N-glycosylation pathways were insensitive to trypsin action and consequently were located in the outer mitochondrial membrane. Based on the activator effect of the trypsin on these enzyme activities, the results suggested two distinct orientations of their active sites. As regards the N-glycoprotein biosynthesis pathway through dolichol intermediates, the dolicholphosphoryl-mannose and dolichol-pyrophosphoryl-di-N-acetylchitobiose synthases would be oriented outside while the oligomannosyl-synthase and the oligomannosyl-transferase would be rather oriented inside in the outer membrane. The N-acetylglucosaminyl-transferase involved in the direct transfer of N-acetylglucosamine from its nucleotide donor to a proteinic acceptor would be oriented outside in the outer membrane.