Membrane markers of endoplasmic reticulum preserved in autophagic vacuolar membranes isolated from leupeptin-administered rat liver

We isolated membranes from leupeptin-induced autophagic vacuoles and compared them with lysosomal membranes purified from dextran-administered rats. In protein composition, autophagic vacuole membranes prepared from long term-starved (36 h) rats bear marked resemblance to lysosomal membranes, whereas vacuole membranes prepared from short term-starved (12 h) animals differ significantly from lysosomal membranes. Immunoblotting analyses showed that only autophagic vacuole membranes from short term-starved rats possess endoplasmic reticulum markers such as cytochrome P450 and NADPH-cytochrome c reductase. None of the membranes contain sialyltransferase, a Golgi membrane marker. In experiments in which rats were starved after feeding to induce autophagy, the appearance of the endoplasmic reticulum markers occurred during 6-12 h of starvation, concomitantly with increases in vacuolar proteins and sequestered cytosolic aldolase. The endoplasmic reticulum membrane markers and sequestered aldolase declined gradually after 20-36 h of starvation, suggesting that prolonged starvation causes no further increase in the formation of autophagic vacuoles but an increase in the population of matured autophagic vacuoles. Thus, the prominent markers of endoplasmic reticulum from which autophagosomes originate are well preserved in autophagic vacuole membranes, and retention of these markers is highly dependent on the formation and subsequent maturation process of autophagic vacuoles.