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© The Rockefeller University Press, 0021-9525/1997//1747 $5.00
The Journal of Cell Biology, Volume 139, Number 7, , 1997 1747-1759

Article

Inhibition of Endosome Function in CHO Cells Bearing a Temperature-sensitive Defect in the Coatomer (COPI) Component {varepsilon}-COP



Elizabeth Daro*, David Sheff*, Marie Gomez{ddagger}, Thomas Kreis{ddagger}, and Ira Mellman*

* Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06520-8002; and {ddagger} Department of Cell Biology, University of Geneva, CH-1211 Geneva 4, Switzerland

Recent evidence has suggested that subunits of the coatomer protein (COPI) complexes are functionally associated with endosomes in mammalian cells. We now provide genetic evidence that COPI plays a role in endocytosis in intact cells. The ldlF mutant CHO cell line bears a temperature-sensitive defect in the COPI subunit {varepsilon}-COP. In addition to exhibiting conditional defects in the secretory pathway, we find that the cells are also defective at mediating endosome-associated functions. As found for cells microinjected with anti-COPI antibodies, ldlF cells at the restrictive temperature could not be infected by vesicular stomatitis (VSV) or Semliki Forest virus (SFV) that require delivery to acidic endosomes to penetrate into the cytosol. Although there was no temperature-sensitive defect in the internalization of receptor-bound transferrin (Tfn), Tfn recycling and accumulation of HRP were markedly inhibited at the restrictive temperature. Sorting of receptor-bound markers such as EGF to lysosomes was also reduced, although delivery of fluid-phase markers was only partially inhibited. In addition, lysosomes redistributed from their typical perinuclear location to the tips of the ldlF cells. Mutant phenotypes began to emerge within 2 h of temperature shift, the time required for the loss of detectable {varepsilon}-COP, suggesting that the endocytic defects were not secondary to a block in the secretory pathway. Importantly, the mutant phenotypes were also corrected by transfection of wild-type {varepsilon}-COP cDNA demonstrating that they directly or indirectly reflected the {varepsilon}-COP defect. Taken together, the results suggest that {varepsilon}-COP acts early in the endocytic pathway, most likely inhibiting the normal sorting and recycling functions of early endosomes.

Abbreviations used in this paper: ARF, ADP ribosylation factor; BFA, brefeldin A; COP, coatomer proteins; LDL, low density lipoprotein; lgp, lysosomal glycoprotein; PNRC, perinuclear recycling compartment; SFV, Semliki Forest virus; Tfn, transferrin; TfnR, transferrin receptor; VSV, vesicular stomatitis virus; LY, Lucifer yellow.

This work was made possible by the generosity and support of M. Krieger (MIT, Cambridge, MA). Funding for D. Sheff provided by the Patrick and Catherine Weldon Donaghue Medical Research Foundation.

E. Daro's present address is Immunex Corporation, 51 University St., Seattle, WA 98110.

Address all correspondence to Ira Mellman, Department of Cell Biology, Yale University School of Medicine, 333 Cedar St., P.O. Box 208002, New Haven, CT 06520-8002. Tel.: (203) 785-5058. Fax: (203) 785-7226. E-mail: ira.mellman{at}yale.edu


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