Pig intestinal microvillus aminopeptidase (EC 3.4.11.2) was reincorporated into lipid membranes by using either beta-octyl glucoside or sodium deoxycholate. The results showed that for this enzyme the deoxycholate-dialysis method was the preferable one. By using this method, microvillus aminopeptidase was inserted almost quantitatively into either phosphatidylcholine vesicles or microvillus-lipid vesicles. By proteolytic treatment of the vesicles, by probing the aminopeptidase with an inhibitory antibody and by monitoring the positioning of the anchor with the aid of [125I]iodonaphthyl azide, it was demonstrated that the catalytically active part was located outside the liposomes and the anchoring peptide(2) was associated with the membrane. Electron-microscopic observation on this model system demonstrated a dimeric symmetrical structure of aminopeptidase (dimensions about 13.5 nm X 5.5 nm) separated by a 5 nm gap from the membrane. This distance corresponds to a molecular weight of 2000-5000 for this junctional segment of the anchor connecting the intramembrane part of the anchor with the catalytically active main part of the aminopeptidase.
