We have previously identified the tumor suppressor in lung cancer 1 (TSLC1) gene as a novel tumor suppressor in human non-small cell lung cancer (NSCLC) by functional complementation. TSLC1 encodes a membrane glycoprotein belonging to an immunoglobulin superfamily and participates in cell adhesion. A truncating mutation of the TSLC1 corresponding to its cytoplasmic domain in a primary NSCLC tumor suggests that this domain is important for tumor suppressor activity. Here, we report that TSLC1 directly associates with MPP3, one of the human homologues of a Drosophila tumor suppressor gene, Discs large (Dlg). This interaction was dependent on the presence of a PDZ-binding motif at the carboxyl terminus of TSLC1. Furthermore, TSLC1 and MPP3 were colocalized at the cell-cell attachment sites in both a low and a high cell density. The MPP3 gene was expressed in normal lung as well as in many tissues examined except for peripheral blood lymphocytes but lost its expression in one of the nine NSCLC cell lines. These results suggest that TSLC1 and MPP3 are involved in the same cascade of cell-cell interaction, and that the disruption of this cascade might lead cells to malignant growth and tumor formation in lung cancer.