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Department of Cancer Biology, The Salk Institute for Biological Studies, San Diego, California 92186-5800
In polarized Madin-Darby canine kidney
(MDCK) cells, the transferrin receptor (TR) is selectively delivered to the basolateral surface, where it internalizes transferrin via clathrin-coated pits and recycles back to the basolateral border. Mutant tailless receptors are sorted randomly in both the biosynthetic
and endocytic pathways, indicating that the basolateral
sorting of TR is dependent upon a signal located within
the 61-amino acid cytoplasmic domain. To identify the
basolateral sorting signal of TR, we have analyzed a series of mutant human TR expressed in MDCK cells.
We find that residues 19-41 are sufficient for basolateral sorting from both the biosynthetic and endocytic
pathways and that this is the only region of the TR cytoplasmic tail containing basolateral sorting information. The basolateral sorting signal is distinct from the YTRF internalization signal contained within this region and is not tyrosine based. Detailed functional analyses of the mutant TR indicate that residues 29-35 are
the most important for basolateral sorting from the biosynthetic pathway. The structural requirements for basolateral sorting of internalized receptors from the endocytic pathway are not identical. The most striking
difference is that alteration of G31DNS34 to YTRF impairs basolateral sorting of newly synthesized receptors
from the biosynthetic pathway but not internalized receptors from the endocytic pathway. Also, mutations have been identified that selectively impair basolateral
sorting of internalized TRs from the endocytic pathway
without affecting basolateral sorting of newly synthesized receptors. These results imply that there are subtle differences in the recognition of the TR basolateral
sorting signal by separate sorting machinery located
within the biosynthetic and endocytic pathways.
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