The integrin beta4 has a long cytodomain necessary for hemidesmosome formation. A yeast two-hybrid screen using beta4 cytodomain uncovered a protein called p27(BBP) that represents a beta4 interactor. Both in yeast and in vitro, p27(BBP) binds the two NH2-terminal fibronectin type III modules of beta4, a region required for signaling and hemidesmosome formation. Sequence analysis of p27(BBP) revealed that p27(BBP) was not previously known and has no homology with any isolated mammalian protein, but 85% identical to a yeast gene product of unknown function. Expression studies by Northern analysis and in situ hybridization showed that, in vivo, p27(BBP) mRNA is highly expressed in epithelia and proliferating embryonic epithelial cells. An antibody raised against p27(BBP) COOH-terminal domain showed that all beta4-containing epithelial cell lines expressed p27(BBP). The p27(BBP) protein is insoluble and present in the intermediate filament pool. Furthermore, subcellular fractionation indicated the presence of p27(BBP) both in the cytoplasm and in the nucleus. Confocal analysis of cultured cells showed that part of p27(BBP) immunoreactivity was both nuclear and in the membrane closely apposed to beta4. These results suggest that the p27(BBP) is an in vivo interactor of beta4, possibly linking beta4 to the intermediate filament cytoskeleton.