Cadherin 23 (Cdh23), an essential factor in inner ear mechano-electric transduction, exists in two alternatively spliced forms, Cdh23(+68) and Cdh23(-68), depending on the presence and absence of exon 68. Cdh23(+68) is inner ear-specific. The exon 68-corresponding region confers an a-helical configuration upon the cytoplasmic domain (Cy) and includes a cysteine residue, Cys(3240). We demonstrate here that Cy(+68) as well as the transmembrane (TM) plus Cy(+68) region is present in two different forms in transfected cells, reduced and non-reduced, the latter existing in more compact configuration than the former. The observed characteristic of Cy(+68) was completely abolished by CYS(3240)Ala substitution. Treatment of TMCy(+68)-transfected cells with diethyl maleate, a glutathione depleting reagent, resulted in conversion of the non-reduced to the reduced form of TMCy(+68), suggesting glutathione to be a Cys(3240) -binding partner. Multiple alignment of mammalian Cdh23Cy sequences indicated the occurrence of conformation-inducible Cys in Cdh23Cy of mammals, but not lower vertebrates. The implications of Cys-dependent structural ambivalence of Cdh23 in inner ear mechanosensation are discussed. (C) 2007 Elsevier Inc. All rights reserved.