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© The Rockefeller University Press, 0021-9525/1999//123 $5.00
The Journal of Cell Biology, Volume 145, Number 1, , 1999 123-139


Regular Articles

The Receptor Recycling Pathway Contains Two Distinct Populations of Early Endosomes with Different Sorting Functions



David R. Sheff, Elizabeth A. Daro, Michael Hull, and Ira Mellman

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.

Key Words: sorting • endosomes • receptor recycling



Abbreviations used in this paper: dIgA, dimeric IgA; EE, early endosome; LE, late endosome; pIgR, polymeric immunoglobulin receptor; RE, recycling endosome; Tfn, transferrin.

Address correspondence to Ira Mellman, Dept. of Cell Biology, Yale University School of Medicine, 333 Cedar St., P.O. Box 208002, New Haven, CT 06520-8002. Tel.: (203) 785-4303 or 4302. Fax: (203) 785-4301. E-mail: ira.mellman{at}yale.edu

Elizabeth A. Daro's current address is Immunex, 51 University St., Seattle, WA 98101.



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