plasticity in the intercalated cell: the hensin pathway. Am. J. Physiol. 275 (Renal Physiol. 44): F183-F190, 1998.-The collecting duct of the renal tubule contains two cell types, one of which, the intercalated cell, is responsible for acidification and alkalinization of urine. These cells exist in a multiplicity of morphological forms, with two extreme types, ␣ and ␤. The former acidifies the urine by an apical proton-translocating ATPase and a basolateral Cl/HCO 3 exchanger, which is an

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... ately spliced form of band 3. This kidney form of band 3, kAE1, is present in the apical membrane of the ␤-cell, which has the H ϩ -ATPase on the basolateral membrane. We had suggested previously that metabolic acidosis leads to conversion of ␤-types to ␣-types. To study the biochemical basis of this plasticity, we used an immortalized cell line of the ␤-cell and showed that these cells convert to the ␣-phenotype when plated at superconfluent density. At high density these cells localize a new protein, which we term "hensin," to the extracellular matrix, and hensin acts as a molecular switch capable of changing the phenotype of these cells in vitro. Hensin induces new cytoskeletal proteins, makes the cells assume a more columnar shape and retargets kAE1 and the H ϩ -ATPase. These recent studies suggest that the conversion of ␤to ␣-cells, at least in vitro, bears many of the hallmarks of terminal differentiation. anion exchanger; proton-ATPase; kanadaptin; acid/base; band 3 Recent studies in nonpolarized cells have shown the existence of "dormant" polarized pathways in that when apical or basolateral proteins were transfected into lymphocytes, osteoclasts, or macrophages, they did not share the same vesicles or routes of traffic (48, 73). However, the provision of external cues, for instance,