We recently reported the isolation and partial characterization of a novel membrane-associated protein designated MAP17. In normal tissues, MAP17 was expressed only in the apical brush border of proximal tubular epithelial cells of the human adult kidney. However, MAP17 was diffusely expressed in most carcinomas originating in the kidney, colon, lung, and breast. Transfection of MAP17 into the HT29 carcinoma cell line markedly decreased cell proliferation in vitro and tumor growth in vivo, suggesting that MAP17 plays a role, either direct or indirect, in the control of cell proliferation. In an attempt to elucidate the function of MAP17, we screened a human kidney cDNA library for interacting proteins using the yeast two-hybrid system and isolated a novel protein containing PDZ protein interaction domains, which we have named PDZK1. PDZK1 is a 519-amino acid protein with a molecular weight of 63 kd; it is expressed in the kidney, pancreas, liver, gastrointestinal tract, and adrenal cortex. In situ hybridization experiments showed that the expression of PDZK1 was limited to epithelial cells. In the kidney, it colocalized with MAP17 in the brush border of proximal tubular epithelial cells. In addition, PDZK1 was overexpressed in selected tumors of epithelial origin. Although the function of PDZK1 has yet to be determined, proteins containing PDZ domains have been shown to play important roles as diverse as cell-cell interaction, cell differentiation, growth control, ion channels organization, and signal transduction. This is of particular interest because MAP17 is localized in areas either of cell-cell contact or where ion channels are localized, for example in the kidney. PDZK1 may represent the link between the cell membrane-where it interacts with MAP17-and other cytoplasmic proteins involved in biologic functions such as cell proliferation, differentiation, and ion transport.