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J. Cell Biol.,
Volume 145, Number 1, April 5, 1999 123-139
Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06520-8002
Receptor recycling involves two endosome
populations, peripheral early endosomes and perinuclear recycling endosomes. In polarized epithelial cells,
either or both populations must be able to sort apical
from basolateral proteins, returning each to its appropriate plasma membrane domain. However, neither the
roles of early versus recycling endosomes in polarity
nor their relationship to each other has been quantitatively evaluated. Using a combined morphological, biochemical, and kinetic approach, we found these two
endosome populations to represent physically and
functionally distinct compartments. Early and recycling
endosomes were resolved on Optiprep gradients and
shown to be differentially associated with rab4, rab11,
and transferrin receptor; rab4 was enriched on early endosomes and at least partially depleted from recycling
endosomes, with the opposite being true for rab11 and
transferrin receptor. The two populations were also
pharmacologically distinct, with AlF4 selectively blocking export of transferrin receptor from recycling endosomes to the basolateral plasma membrane. We applied
these observations to a detailed kinetic analysis of
transferrin and dimeric IgA recycling and transcytosis.
The data from these experiments permitted the construction of a testable, mathematical model which enabled a dissection of the roles of early and recycling endosomes in polarized receptor transport. Contrary to
expectations, the majority (>65%) of recycling to the
basolateral surface is likely to occur from early endosomes, but with relatively little sorting of apical from
basolateral proteins. Instead, more complete segregation of basolateral receptors from receptors intended
for transcytosis occurred upon delivery to recycling endosomes.
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