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A similar function has been previously suggested for the 56-kDa subunit (B1), which was detected in nonacidifying endosomes isolated from kidney collecting duct principal cells. Alternatively, some unidentified V0 domain subunit isoforms may be expressed in principal cells and may serve to anchor dl in their apical membrane. Potential Role of Different Subunit Isoforms of the V-ATPase in Epididymal Epithelial Cells The present study shows that different isoforms of the same subunit of both the V0 and the V1 domains colocalize in the apical pole of narrow and clear cells (Cl and C2, Gl and G2, a1 and a4, d1 and d2). In yeast and osteoclasts, targeting of the V-ATPase is controlled by the assembly of different a subunit isoforms into the holoenzyme. The three a isoforms detected in clear cells may, therefore, be involved in differential targeting of the V-ATPase to specific membrane domains: a2 in TGN-associated structures, a1 and a4 in endosomes, and a4 in apical microvilli. Previous studies have shown that subunits d, C, and G participate in the stability of the V-ATPase holoenzyme, and these subunits may also play a role in stabilizing the V-ATPase on specific membrane domains in narrow and clear cells.