Localization of membrane-associated guanylate kinase (MAGI)-1/BAI-associated protein (BAP) 1 at tight junctions of epithelial cells

Membrane-associated guanylate kinase (MAGI)-1/BAI-associated protein (BAP) 1 and Synapse-associated protein (SAP) 97/human Discs-large tumor suppresso r gene (hDLG) are ubiquitous isoforms of synaptic scaffolding molecule (S-S CAM) and Postsynaptic density(PSD)-95/SAP90, both of which are implicated i n the structures of synapses, respectively. SAP97/hDLG is localized at epit helial junctions and may function as a scaffolding protein, but the subcell ular localization or the function of MAGI-1/BAP1 has not been clarified. In intestinal epithelial cells, MAGI-1/BAP1 was localized at tight junctions, whereas SAP97/hDLG was localized diffusely at cell-cell junctions. In Madi ne Darby canine kidney (MDCK) cells, MAGI-1/BAP1 was colocalized with ZO-1, whereas SAP97/hDLG was colocalized with E-cadherin. In MDCK cells, dominan t active and negative mutants of Rad 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 Ca2+ medium, E-cadherin disappeared from the p lasma membrane, and cells were dissociated. The phorbol 12-myristate 13-ace tate-treatment after the low Ca2+ switch induced a tight junction-like stru cture. MAGI-1/BAP1 was recruited with ZO-1 to this structure, but SAP97/hDL G or E-cadherin was not. These findings suggest that MAGI-1/BAP1 is a compo nent of tight junctions of epithelial cells, and that its role is different from that of SAP97/hDLG.