A basolateral sorting signal is encoded in the α-subunit of Na-K- ATPase

Na-K-ATPase and H-K-ATPase are highly homologous ion pumps that exhibit distinct plasma membrane distributions in epithelial cells. We have studied the α-subunits of these heterodimeric pumps to identify the protein domains responsible for their polarized sorting. A chimeric α-subunit construct (N519H) was generated in which the first 519 amino acid residues correspond to the Na-K-ATPase sequence and the remaining 500 amino acids are derived from the H-K-ATPase sequence. In stably transfected LLC-PK₁ cell lines, we found that the N519H chimera is restricted to the basolateral surface under steady-state conditions, suggesting that residues within the NH₂-terminal 519 amino acids of the Na-K-ATPase α-subunit contain a basolateral sorting signal. H-K-ATPase β-subunit expressed alone in LLC-PK₁ cells accumulates at the apical surface. When coexpressed with N519H, the H-K-ATPase β- subunit assembles with this chimera and accompanies it to the basolateral surface. Thus the NH₂-terminal basolateral signal in the Na-K-ATPase α- subunit masks or is dominant over any apical sorting information present in the β-polypeptide. In gastric parietal cells, the H-K-ATPase β-subunit targets the H-K-ATPase to an intracellular vesicular compartment which fuses with the plasma membrane in response to secretagogue stimulation. To test whether the chimera-H-KATPase β-subunit complex is directed to a similar compartment in LLC-PK₁ cells, we treated transfected cells with drugs that raise intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels. Elevation of cytosolic cAMP increased the surface expression of both the N519H chimera and the H-K-ATPase β-subunit. This increase in surface expression, however, appears to be the result of transcriptional upregulation and not recruitment of chimera to the surface from a cAMP-inducible compartment.