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Differential Recognition of a Tyrosine-Dependent Signal in the Basolateral and Endocytic Pathways of Thyroid Epithelial Cells

Centro di Endocrinologia ed Oncologia Sperimentale del Centro Nazionale delle Ricerche-Dipartimento di Biologia e Patologia Cellulare e Molecolare (C.L., G.R., L.P., L.N., C.Z.), Università degli Studi di Napoli "Federico II," 80131 Napoli, Italy; and Institut de Biologie du Développement de Marseille (L.M., A.L.), Université de la Méditerranée, Faculté des Sciences de Luminy, Marseille, France

Address all correspondence and requests for reprints to: Chiara Zurzolo, Dipartimento di Biologia e Patologia Cellulare e Molecolare, II Facoltà di Medicina, Via Pansini 5, 80131 Napoli, Italy. E-mail: . zurzolo{at}ds.unina.it

Trafficking of receptors is of crucial importance for the physiology of most exocrine and endocrine organs. It is not known yet if the same mechanisms are used for sorting in the exocytic and endocytic pathways in the different epithelial tissues. In this work, we have used a deletion mutant of the human neurotrophin receptor p75^hNTR that is normally localized on the apical membrane when expressed in Madin-Darby canine kidney cells. This internal 57-amino acid deletion of the cytoplasmic tail leads to a relocation of the protein from the apical to the basolateral membrane and to rapid and efficient endocytosis. These events are mediated by a signal localized within 9 amino acids of the mutated cytoplasmic tail that is strictly dependent on a tyrosine residue (Tyr-308). We have analyzed the basolateral sorting efficiency and endocytic capacity of this signal in Fischer rat thyroid (FRT) cells, in which basolateral and endocytic determinants have not yet been identified. We found that this targeting signal can mediate efficient transport to the basolateral membrane also in FRT cells with similar tyrosine dependence as in MDCK cells. In contrast to MDCK cells, this Tyr-based signal was not able to mediate coated pits localization and endocytosis in FRT cells. These data represent the first characterization of basolateral/endocytic signals in thyroid epithelial cells. Furthermore, our results indicate that requirements for tyrosine-dependant basolateral sorting signals are conserved among cell lines from different tissues but that the recognition of the colinear endocytic signal is tissue specific.

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