Two distinct kinds of tubular organelles involved in the rapid recycling and slow processing of endocytosed transferrin.

The tubular structures of endosomes are thought to mediate the sorting and recycling of endocytosed macromolecules. These structures have been reported to show considerable morphological variety. However, it is not clear whether they are functionally identical. To address this question, we applied quantitative imaging analysis to characterize tubular organelles loaded with a recycling protein marker, fluorescent transferrin, in living human carcinoma HEp2 cells, using laser scanning confocal microscopy. High-resolution images of the cells demonstrated two types of tubular structures with a distinct morphology and showing a time dependency in their appearance: the fine tubular element and the extensive tubular element. Fine tubular elements 2-10 microns long were distributed throughout the cytoplasm after 10 min of loading with the tracer. Extensive tubular elements 5-20 microns long radiated from the cytocenter after 2 h of loading, but not after 10 min. Time-lapse imaging analysis demonstrated that the half-life of transferrin in the fine and extensive tubular elements was 12 min and approximately 50 min, respectively, at 33 degrees C. Double labeling experiments using fluorescent transferrin and epidermal growth factor indicated that the extensive tubular element was neither a late endosome nor a lysosome. From these results, we conclude that the fine tubular and extensive tubular elements are distinct organelles: the former comprising the sorting endosome and recycling compartment which mediate the rapid recycling of transferrin, and the latter being part of a novel pathway of slower transferrin processing.