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Cover  By far the best understood mechanism of endocytosis is via clathrin-coated pits. It has been appreciated for many years that there may be one or more mechanisms for endocytosis that are not mediated by clathrin. Non-clathrin endocytosis has moved to the fore because of several recent developments, such as the use of a mutant form of dynamin to inhibit clathrin-dependent endocytosis.

An article by Hansen et al. (J. Cell Biol. [1991]. 113, 731-741) was one of the first papers to provide definitive evidence for the existence of non-clathrin endocytosis and is still one of the best morphological analyses of this process. The authors used a clever scheme to label very early ("primary") endocytic vesicles and show by serial sectioning that they are truly not in continuity with the cell surface.

HEp-2 cells were incubated at 4°C with Concanavalin A (Con A) coupled to 5-nm gold particles. Subsequently, the cells were warmed to 37°C for 30 or 60 s to allow endocytosis of the marker before fixation in ice-cold fixative to immediately arrest the internalization process. Next, cell surface-associated and internalized Con A-gold were distinguished by incubating the cells with an HRP-tagged antibody to Con A and performing a diaminobenzidine reaction before processing the cells for electron microscopy. Figures 5 and 6 show two examples of non-clathrin-coated vesicles containing Con A-gold complexes as revealed by thin serial sectioning (arrows in Fig. 5, b-e, and 6, d-f (double arrows), another non-clathrin-coated vesicle is appearing. The figure furthermore validates that vesicles containing Concanavalin A-gold complexes devoid of surrounding HRP labeling are truly primary endocytic vesicles. This allowed for quantitations demonstrating that the average size of non-clathrin-coated endocytic vesicles in this assay was 95 nm, somewhat smaller than clathrin-coated vesicles, and that these vesicles appeared with a frequency approximately equal to that of clathrin-coated vesicles. These vesicles are not clathrin coated and are not caveolae. Reproduced from The Journal of Cell Biology, 1991, 113, 731-741, by copyright permission of The Rockefeller University Press.Keith Mostov