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J. Cell Biol.,
Volume 142, Number 5, September 7, 1998 1245-1256
* Department of Ophthalmology, All basolateral sorting signals described to
date reside in the cytoplasmic domain of proteins,
whereas apical targeting motifs have been found to be
lumenal. In this report, we demonstrate that wild-type
rhodopsin is targeted to the apical plasma membrane via the TGN upon expression in polarized epithelial
MDCK cells. Truncated rhodopsin with a deletion of 32 COOH-terminal residues shows a nonpolar steady-state distribution. Addition of the COOH-terminal 39 residues of rhodopsin redirects the basolateral membrane protein CD7 to the apical membrane. Fusion of
rhodopsin's cytoplasmic tail to a cytosolic protein glutathione S-transferase (GST) also targets this fusion
protein (GST-Rho39Tr) to the apical membrane. The
targeting of GST-Rho39Tr requires both the terminal
39 amino acids and the palmitoylation membrane anchor signal provided by the rhodopsin sequence. The
apical transport of GST-Rho39Tr can be reversibly
blocked at the Golgi complex by low temperature and
can be altered by brefeldin A treatment. This indicates
that the membrane-associated GST-Rho39Tr protein
may be sorted along a yet unidentified pathway that is
similar to the secretory pathway in polarized MDCK
cells. We conclude that the COOH-terminal tail of
rhodopsin contains a novel cytoplasmic apical sorting
determinant. This finding further indicates that cytoplasmic sorting machinery may exist in MDCK cells for
some apically targeted proteins, analogous to that described for basolaterally targeted proteins.
Department of Cell Biology and Anatomy, The Margaret M. Dyson Vision Research Institute,
Cornell University Medical College, New York 10021
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