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J. Cell Biol.,
Volume 141, Number 1, April 6, 1998 115-133
* Department of Cell Biology and Anatomy, The Johns Hopkins School of Medicine, Baltimore, Maryland 21205; and We studied basolateral-to-apical transcytosis of three classes of apical plasma membrane (PM)
proteins in polarized hepatic WIF-B cells and then
compared it to the endocytic trafficking of basolaterally
recycling membrane proteins. We used antibodies to label the basolateral cohort of proteins at the surface of living cells and then followed their trafficking at 37°C
by indirect immunofluorescence. The apical PM proteins aminopeptidase N, 5'nucleotidase, and the polymeric IgA receptor were efficiently transcytosed. Delivery to the apical PM was confirmed by microinjection
of secondary antibodies into the bile canalicular-like
space and by EM studies. Before acquiring their apical
steady-state distribution, the trafficked antibodies accumulated in a subapical compartment, which had a unique tubulovesicular appearance by EM. In contrast,
antibodies to the receptors for asialoglycoproteins and
mannose-6-phosphate or to the lysosomal membrane
protein, lgp120, distributed to endosomes or lysosomes,
respectively, without accumulating in the subapical area. However, the route taken by the endosomal/lysosomal protein endolyn-78 partially resembled the transcytotic pathway, since anti-endolyn-78 antibodies
were found in a subapical compartment before delivery
to lysosomes. Our results suggest that in WIF-B cells,
transcytotic molecules pass through a subapical compartment that functions as a second sorting site for a
subset of basolaterally endocytosed membrane proteins
reaching this compartment.
Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218
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