Transglutaminase type 1 and its cross-linking activity are concentrated at adherens junctions in simple epithelial cells

Transglutaminase type 1 was identified as a tyrosinephosphorylated protein from the isolated junctional fraction of the mouse liver. This enzyme was reported to be involved in the covalent cross-linking of proteins in keratinocytes, but its expression and activity in other cell types have not been examined. Northern blotting revealed that transglutaminase type 1 was expressed in large amounts in epithelial tissues (lung, liver, and kidney), which was also confirmed by immunoblotting with antibodies raised against mouse recombinant protein. Immunoblotting of the isolated junctional fraction revealed that transglutaminase type 1 was concentrated in the fraction not only as a 97-kDa form but also as forms of various molecular masses cross-linked to other proteins. In agreement with this finding, endogenous transglutaminase type 1 was immunofluorescently colocalized with E-cadherin in cultured simple epithelial cells. In the liver and kidney, immunoelectron microscopy revealed that transglutaminase type 1 was concentrated, albeit not exclusively, at cadherin-based adherens junctions. Furthermore, by in vitro and in vivo labeling, transglutaminase cross-linking activity was also shown to be concentrated at intercellular junctions of simple epithelial cells. These findings suggested that the formation of covalently cross-linked multimolecular complexes by transglutaminase type 1 is an important mechanism for maintenance of the structural integrity of simple epithelial cells, especially at cadherin-based adherens junctions.