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Because of its brighter apical staining in clear cells and its high expression in kidney intercalated cells, d2 appears to be the predominant isoform in proton-secreting narrow and clear cells. Principal Cell Localization Interestingly, subunit A was detected in principal cells, where it was localized in intracellular structures closely associated with but not identical to the TGN38-labeled trans-Golgi network (TGN). Immunogold electron microscopy will be required to identify precisely these intracellular structures. Clear cells also showed a similar staining pattern for subunit A, but the staining was much weaker compared to principal cells. In addition, isoform a2 was detected in similar intracellular structures in principal cells with a weaker staining detected in clear cells. Staining of serial sections with an anti p-COP antibody showed no apparent colocalization of a2 with p-COP in the Golgi apparatus (data not shown). Localization of a2 protein in both clear and principal cells correlates with the previously described ubiquitous distribution of a2 mRNA. These results indicate that subunits A and a2 may be involved in the acidification of intracellular organelles in both clear and principal cells.