MAGI-1: a widely expressed, alternatively spliced tight junction protein.

Tight junctions are apically localized structures that regulate the passage of small molecules and proteins through intercellular regions of epithelial or endothelial cells. These structures are complex multimolecular assemblages that contain both transmembrane and membrane-associated proteins. MAGUKs (Membrane-Associated Guanylate Kinases) are a family of scaffolding proteins that contain multiple protein interaction domains, including PDZ, SH3, WW, and guanylate kinase motifs, and have been grouped into five discrete subfamilies based on homology. Little is known regarding the most recently described subfamily of MAGUKs, termed MAGIs (MAGUKS with Inverted domain structure). Here we show that two of the three known MAGI isoforms, MAGI-1 and MAGI-3, are present in the tight junctions of cultured epithelial cells. A broader examination of MAGI-1 expression in vivo shows that it is present in the tight junctions of all epithelial cell types examined. Human MAGI-1 transcripts are alternatively spliced at three sites, and two forms are expressed only in nonepithelial tissues, predominantly in brain. The major form that is expressed in cultured colon carcinoma epithelial cells, as well as several epithelial-rich tissues, contains an extended carboxy terminus encoding potential nuclear targeting signals. MAGI-1, ZO-1, and ZO-2 all colocalize in nonpolarized epithelial cells, suggesting that they form a preassembled complex that is incorporated into the tight junction upon polarization. Finally, all of the alternatively spliced forms of MAGI-1 show tight junction localization, and this localization occurs in the absence of the guanylate kinase and WW domains as well as the extended carboxy terminus.