Cycling of the integral membrane glycoprotein, LEP100, between plasma membrane and lysosomes: Kinetic and morphological analysis

LEP100 (an integral membrane glycoprotein, M r = 100,000) occurs in three subcellular compartments: lysosome (approximately 90% of the molecules), endosome (5%–8%), and plasma membrane (2%–3%). Rate constants for movement to and from each compartment have been estimated. The movement of LEP-100 from endosomes to lysosomes was blocked by chloroquine, causing redistribution to a new steady state in which about 30% of LEP100 molecules were localized in clathrin-coated patches on the cell surface, while intracellular LEP100 occurred in nearby endocytic vesicles. The cell-surface and endosomal pools of LEP100 remained in rapid equilibrium ( t 1 2 about 5 min). These results support the existence of a hitherto unappreciated pathway of membrane flow from lysosomes. The lysosome should not be considered simply a terminal target of membrane trafficking.