Membrane-associated guanylate kinase (MAGI)-1/BAIassociated protein (BAP) 1 and Synapse-associated protein (SAP) 97/human Discs-large tumor suppressor gene (hDLG) are ubiquitous isoforms of synaptic scaolding molecule (S-SCAM) and Postsynaptic density (PSD)-95/SAP90, both of which are implicated in the structures of synapses, respectively. SAP97/hDLG is localized at epithelial junctions and may function as a scaolding protein, but the subcellular localization or the function of MAGI-1/BAP1 has not been clari®ed. In intestinal epithelial cells, MAGI-1/BAP1 was localized at tight junctions, whereas SAP97/hDLG was localized diusely at cell ± cell junctions. In Madine Darby canine kidney (MDCK) cells, MAGI-1/BAP1 was colocalized with ZO-1, whereas SAP97/hDLG was colocalized with E-cadherin. In MDCK cells, dominant active and negative mutants of Rac1 small G protein changed the amounts of SAP97/hDLG at cell ± cell junctions, but not that of MAGI-1/BAP1. When MDCK cells were switched to a low Ca 2+ medium, E-cadherin disappeared from the plasma membrane, and cells were dissociated. The phorbol 12-myristate 13-acetate-treatment after the low Ca 2+ switch induced a tight junction-like structure. MAGI-1/BAP1 was recruited with ZO-1 to this structure, but SAP97/hDLG or E-cadherin was not. These ®ndings suggest that MAGI-1/BAP1 is a component of tight junctions of epithelial cells, and that its role is dierent from that of SAP97/hDLG.