Slowed receptor trafficking in mutant CHO lines of the end1 and end2 complementation groups

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Abstract
A mutant Chinese hamster ovary cell line with nonconditional kinetic defects in receptor internalization and recycling was isolated, based on selection for resistance to a transferrin-diphtheria toxin conjugate and screening for aberrant receptor trafficking. The 12-4 cell line internalizes transferrin at ∼75% of the parental rate and recyles transferrin back to the cell surface at ∼55% of the parental rate. Internalization of low density lipoprotein is also reduced to ∼70% of the parental cell rate, demonstrating that the mutant phenotype affects the trafficking of multiple receptors. Characterization of somatic cell hybrids indicated that the 12-4 phenotype is recessive, and complementation analysis determined that the 12-4 cell line is a member of the End2 complementation group. End2 mutants have previously been described as defective in endosomal acidification but have not been known to be defective in receptor trafficking. We have found similar defects in another End2 mutant cell line, suggesting that slowed receptor trafficking is characteristic of End2 mutants. Interestingly, transferrin receptor recycling and internalization are also slowed in another complementation group of mutants, End1, that is also defective in endosomal acidification. This study demonstrates altered receptor trafficking in End1 and End2 cell lines, a novel aspect of the mutant phenotypes. These findings provide evidence, based on a cellular genetic approach, that proper endosome acidification is necessary for maintenance of normal receptor recycling.

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