Catenins, a family of structurally related proteins, are involved in epidermal keratinocyte cell-cell adhesion by interacting through their central Armadillo repeats with the intracellular domains of cadherins, transmembrane components of the adhesion junctions. p120 ctn is a catenin expressed in different isoforms due to alternative splicing and multiple translation start sites. BP180 is a collagenous transmembrane protein (type XVII collagen) localized to hemidesmosomal attachment complexes in basal keratinocytes. In this study, we have delineated the molecular interaction between these two proteins utilizing the yeast two-hybrid system, which was confirmed by an in vitro protein-protein interaction assay. Specifically, it was shown that an amino-terminal segment of BP180 (aa. 13-25) contains the information necessary for binding to p120 ctn isoforms 1-3, but not to the isoform 4, suggesting that the interacting domain is located immediately upstream from the Armadillo repeats and is encoded by exons 5 and 6, which are subject to alternative splicing only in a minority of transcripts. In addition to epidermal keratinocytes, p120 ctn was shown to be expressed in a variety of adult and fetal tissues as well as in a number of human tumors. The expression pattern of various p120 ctn transcripts, reflecting alternative splicing of the 5ะ exons, was strikingly similar between the corresponding adult and fetal tissues, while the expression patterns were discordant between certain tumors and their normal parental tissues, suggesting a functional role for the tissue-specific expression of the p120 ctn isoforms. Finally, the tissue-specific expression of BP180 was shown to partially overlap with that of p120 ctn , suggesting that the interaction of these two proteins may contribute to the modulation of cell-cell/matrix interactions in such tissues.