Differential localization of human nongastric H+-K+-ATPase ATP1AL1 in polarized renal epithelial cells

The human H+-K+-ATPase, ATP1AL1, belongs to the subgroup of nongastric, K+- transporting ATPases. In concert with the structurally related gastric H+-K +-ATPase, it plays a major role in K+ reabsorption in various tissues, incl uding colon and kidney. Physiological and immunocytochemical data suggest t hat the functional heteromeric ion pumps are usually found in the apical pl asma membranes of renal epithelial cells. However, the low expression level s of characteristic nongastric ion pumps makes it difficult to verify their spatial distribution in vivo. To investigate the sorting behavior of ATP1A L1, we expressed this pump by stable transfection in MDCK and LLC-PK1 renal epithelial cell lines. Stable interaction of ATP1AL1 with either the endog enous Na+-K+-ATPase beta-subunit or the gastric H+-K+-ATPase beta-subunit w as tested by confocal immunofluorescence microscopy and surface biotinylati on. In cells transfected with ATP1AL1 alone, the alpha-subunit accumulated intracellularly, consistent with its inability to assemble and travel to th e plasma membrane with the endogenous Na+-K+-ATPase beta-subunit. Cotransfe ction of ATP1AL1 with the gastric H+-K+-ATPase beta-subunit resulted in pla sma membrane localization of both pump subunits. In cotransfected MDCK cell s the heteromeric ion pump was predominantly polarized to the apical plasma membrane. Functional expression of ATP1AL1 was confirmed by Rb-86(+) uptak e measurements. In contrast, cotransfected LLC-PK1 cells accumulate ATP1AL1 at the lateral membrane. The distinct polarization of ATP1AL1 indicates th at the a-subunit encodes sorting information that is differently interprete d by cell type-specific sorting mechanisms.