Subcellular distribution of tissue kallikrein and Na,K-ATPase alpha-subunit in rat parotid striated duct cells.

Intracellular protein distribution and sorting were examined in rat parotid striated duct cells, in which tissue kallikrein is apical, and Na,K-ATPase is basolateral. Electron-microscopic immunogold cytochemistry, with both polyclonal and monoclonal antibodies, demonstrated these enzymes at opposite poles of the cells and in distinct intracellular sites. Kallikrein was found within apical secretory granules, whereas Na,K-ATPase was present on basolateral cell membranes. In addition, kallikrein was localized throughout cisternae of all Golgi profiles, whereas Na,K-ATPase (alpha-subunit) was found only in small peripheral vesicles and/or lateral cisternal extensions of a basal subset of Golgi profiles. These differences in the subcellular distribution of the two marker antigens were most clearly seen with double immunogold labelling. Our results suggest that kallikrein, an apical, regulated secretory protein, and Na,K-ATPase, a basolateral, constitutively transported membrane protein, are segregated at (or prior to) the level of the Golgi apparatus rather than in the trans-Golgi network (TGN), as was expected.