Immunoelectron microscopic localization of protein 4.1B in proximal S1 and S2 tubules of rodent kidneys.

Protein 4.1 family proteins are supposed to interact with intramembranous proteins and membrane skeletons. Protein 4.1B, one of the family proteins, was recently reported to be localized in basolateral regions of mouse renal proximal tubules. In this study, we extended the idea that protein 4.1B may be related to ion balance in the region by immunohistochemical studies by light and electron microscopy with our antibody against protein 4.1B. Protein 4.1B distribution in rodent kidneys was determined by comparing with lectin Lotus tetragonobulus agglutinin (LTA), a proximal tubule cell marker, and also with Na(+)/HCO(3)(-)-cotransporter-1 (NBC-1), being expressed in basolateral domains of segment S1 to S2 proximal tubule epithelial cells. Specific protein 4.1B immunolabeling was observed in cuboidal epithelial cells basolaterally, starting their localization from a urinary pole at the glomerulus, whereas the squamous epithelial cells of Bowman's capsules were immunonegative. Rat Bowman's capsules had no simple cuboidal cells, where no protein 4.1B immunostaining appeared. All the protein 4.1B-positive epithelial cells were LTA positive. By immunoelectron microscopy, protein 4.1B immunolabeling of the proximal epithelial cells was restricted to the basolateral membranes, including basal infoldings, whereas tight junctions were not immunolabeled. It is concluded that protein 4.1B might play a role related to membrane skeletal proteins in the basolateral membranes of S1 and S2 proximal tubule cells. Moreover, the immunolocalization of protein 4.1B was almost the same as that of NBC-1, indicating a possible function as a regulator of ion balance, such as Na(+) and HCO(3)(-) reabsorption.