Induced pinocytosis and endosomal pathways in Amoeba proteus

The mode of endosome formation and processing was studied in Amoeba proteus by using bovine serum albumin (BSA) either coupled to the fluorochrome TRITC or to 12 nm gold particles (Au(12)) as a pinocytosis inducer. The intraendosomal modifications of BSA-TRITC and BSA-Au(12) start with the separation of the ligand complex from receptor sites of the endosomal membrane obviously caused by acidification (1-15 min). Subsequently, the delivery with lysosomal enzymes and the disintegration of the ligand complex into individual components (BSA and Au(12)) occurs (15-30 min). The intracytotic transport of the ligand components to distinct cellular compartments leads to separate endosome populations with a common fate, i.e. fusion with the cell membrane and exocytosis of the lysosomal content. However, a certain amount of vesicles containing the hydrolyzable ligand BSA escapes exocytosis and is delivered to the digestive apparatus thus contributing to the nutrient pool of the cell (30-40 min). The application of antibodies against human or Tetrahymena digestive enzymes resulted in the detection of a 22 kD protein which shows a distinct cross-reactivity with anti-cathepsin D. The protein is localized in the Golgi apparatus, in small vesicles and in secondary lysosomes containing ingested material. In general, induced pinocytosis can mainly be viewed as a survival mechanism developed by A. proteus to overcome extracellular stress situations. The consecutive endocytotic pathways involve the lysosomal system as well as some other intracellular compartments and are comparable to the situation in higher organisms.