Nonclathrin‐coated vesicles are involved in endocytosis in kidney collecting duct intercalated cells

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Abstract
Intercalated cells of the kidney collecting duct are able to modify the structure of their apical plasma membrane in response to different physiological conditions. It has been proposed that this process involves the transfer of membrane components (including a proton-pumping ATPase) to and from the apical membrane by a specialized population of tubulovesicles that are found in the apical cytoplasm of these cells. These vesicles have a prominent cytoplasmic coat of regularly arranged dense studs that we have recently shown to be immunocytochemically and morphologically distinct from clathrin. In this study, we have examined the function of these vesicles by using horseradish peroxidase as a tracer of endocytosis at the light and electron microscopic levels. Following the intravenous injection of rats with the tracer, we found a massive labeling of the tubulovesicle compartment of intercalated cells, providing direct evidence that these nonclathrin-coated vesicles are involved in endocytotic events in this cell type. This novel membrane coating material could contain the cytoplasmic domains of molecules transported to and from the plasma membrane by these vesicles (e.g., an H+ ATPase) or it could be a molecule that is involved in vesicle function, by analogy with clathrin.