Immunolocalization of 15-kDa membrane proteins in the kidneys of normal and acidotic rats.

Proteins with apparent molecular masses between 15 kDa and 17 kDa were enriched from rat renal brush-border membranes by preparative gel electrophoresis and used for immunization of rabbits. The serum of one of the rabbits reacted in Western blots of separated renal brush-border proteins with a single 15-kDa band. A comparably strong reaction is seen with a 15-kDa band of renal endosomal proteins. Basolateral membranes show a much weaker reaction. In light- and electron-microscopic studies the serum stains brush-border membranes and endosomes in rat proximal tubule cells, but not mitochondria and basolateral membranes. In cortical collecting ducts, principal cells are not stained with the antiserum. alpha-type (H(+)-secreting) intercalated cells bind the antibodies at apical tubulovesicles. The luminal membrane is scarcely labelled. Conversely, beta-type (HCO3(-)-secreting) intercalated cells exhibit antibody binding to their basolateral membrane. Thus, the antiserum detects 15-kDa proteins differently sorted in alpha- and beta-intercalated cells. After induction of an acute (6 h) metabolic acidosis, the antibody-binding pattern changes only in intercalated cells, type alpha, and occurs at the markedly enlarged luminal plasma membrane. The amount of alpha-type intercalated cells with enlarged luminal membrane ("secreting cell") increases at the expense of alpha cells with apical tubulovesicles ("resting cell"). Taken together, the antiserum detects 15-kDa proteins, the localization and adaptive changes to metabolic acidosis of which are similar to H(+)-ATPases. The functional role of the 15-kDa proteins needs to be established in further studies.