Identification of isoforms of G proteins and PKC that colocalize with tight junctions

Recent evidence suggests that the formation and permeability of tight junctions are actively regulated by second-messenger-generating systems involving G proteins and protein kinase C (PKC). A possible specific target for these regulatory proteins is the tight junction protein ZO-1. An extensive immunocytochemical study was performed in cultured epithelial monolayers of MDCK and Caco-2 cells to identify which isoforms of G proteins and PKC are present at or near the zonula occludens complex. Antibodies against a-subunits of each one of the four major subfamilies were used for the localization of the G proteins. For the PKC localization, antibodies against eight different isoforms were used. In confluent monolayers, G alpha(12) and PKC xi were the only isoforms of these proteins present at the cell borders. In subconfluent monolayers, G alpha(12) and PKC xi were found at the plasma membrane only along the areas of lateral cell-cell contact. These isoforms formed a pattern of distribution very similar to the ZO-1 protein. The present findings indicate that G alpha(12) and PKC xi may be part of the zonula occludens complex and may locally regulate formation and permeability of tight junctions.