THE CYTOPLASMIC TAIL OF RHODOPSIN ACTS AS A NOVEL APICAL SORTING SIGNAL IN POLARIZED MDCK CELLS

All basolateral sorting signals described to date reside in the cytopl asmic domain of proteins, whereas apical targeting motifs have been fo und to be lumenal. In this report, we demonstrate that wild-type rhodo psin is targeted to the apical plasma membrane via the TGN upon expres sion in polarized epithelial MDCK cells. Truncated rhodopsin with a de letion of 32 COOH-terminal residues shows a nonpolar steady-state dist ribution. Addition of the COOH-terminal 39 residues of rhodopsin redir ects the basolateral membrane protein CD7 to the apical membrane. Fusi on 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 te rminal 39 amino acids and the palmitoylation membrane anchor signal pr ovided 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 membr ane-associated GST-Rho39Tr protein may be sorted along a yet unidentif ied 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 i ndicates that cytoplasmic sorting machinery may exist in MDCK cells fo r some apically targeted proteins, analogous to that described for bas olaterally targeted proteins.