Molecular Sorting in Early Endosomes
The early endosome is an intracellular membrane organelle that specializes in the sorting and recycling of endocytosed macromolecules. To elucidate the molecular sorting reaction, early endosomes in living human epidermal carcinoma A431 cells were stained with fluorescently-labeled transferrin and individual endosomes were continuously observed under a real-time con focal microscope. Early endosomes were observed as organelles with various different shapes, such as vesicles, tubules and complexes of these. However, the accumulation of sorted molecules and fission of the endosomal membrane, which are essential for molecular sorting, were predominantly observed in tubular endosomes. These observations indicate that tubular endosomes are key elements for the molecular sorting reaction.