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*Laboratory of Epithelial Cell Biology, Renal-Electrolyte Division,
University of Pittsburgh, Pittsburgh, Pennsylvania 15261;
The function of acidification along the endocytic pathway is not
well understood, in part because the perturbants used to modify
compartmental pH have global effects and in some cases alter
cytoplasmic pH. We have used a new approach to study the effect of pH
perturbation on postendocytic traffic in polarized Madin-Darby canine
kidney (MDCK) cells. Influenza M2 is a small membrane protein that
functions as an acid-activated ion channel and can elevate the pH of
the trans-Golgi network and endosomes. We used
recombinant adenoviruses to express the M2 protein of influenza virus
in polarized MDCK cells stably transfected with the polymeric
immunoglobulin (Ig) receptor. Using indirect immunofluorescence and
immunoelectron microscopy, M2 was found to be concentrated at the
apical plasma membrane and in subapical vesicles; intracellular M2
colocalized partly with internalized IgA in apical recycling endosomes
as well as with the trans-Golgi network marker TGN-38. Expression of M2 slowed the rate of IgA transcytosis across polarized MDCK monolayers. The delay in transport occurred after IgA reached the
apical recycling endosome, consistent with the localization of
intracellular M2. Apical recycling of IgA was also slowed in the
presence of M2, whereas basolateral recycling of transferrin and
degradation of IgA were unaffected. By contrast, ammonium chloride
affected both apical IgA and basolateral transferrin release. Together,
our data suggest that M2 expression selectively perturbs acidification
in compartments involved in apical delivery without disrupting other
postendocytic transport steps.
Department of Anatomy, University of California, San
Francisco Medical School, San Francisco, California 94143; and
Cell Genesys, Foster City, California 94404
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