LC3, a mammalian homologue of yeast Apg8p, is localized in autophagosome membranes after processing

Little is known about the protein constituents of autophagosome membranes i n mammalian cells, Here we demonstrate that the rat microtubule-associated protein 1 light chain 3 (LC3), a homologue of Apg8p essential for autophagy in yeast, is associated to the autophagosome membranes after processing, T wo forms of LC3, called LC3-I and -II, were produced post-translationally i n various cells. LC3-I is cytosolic, whereas LC3-II is membrane bound, The autophagic vacuole fraction prepared from starved rat liver was enriched wi th LC3-II, Immunoelectron microscopy on LC3 revealed specific labelling of autophagosome membranes in addition to the cytoplasmic labelling, LC3-II wa s present both inside and outside of auto-phagosomes, Mutational analyses s uggest that LC3-I is formed by the removal of the C-terminal 22 amino acids from newly synthesized LC3, followed by the conversion of a fraction of LC 3-I into LC3-II, The amount of LC3-II is correlated with the extent of auto phagosome formation. LC3-II is the first mammalian protein identified that specifically associates with autophagosome membranes.