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Am J Physiol Cell Physiol 274: C688-C696, 1998;
0363-6143/98 $5.00
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Vol. 274, Issue 3, C688-C696, March 1998

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

T. R. Muth1, C. J. Gottardi1, D. L. Roush1, and M. J. Caplan2

Departments of 1 Cell Biology and 2 Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510

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 alpha -subunits of these heterodimeric pumps to identify the protein domains responsible for their polarized sorting. A chimeric alpha -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 alpha -subunit contain a basolateral sorting signal. H-K-ATPase beta -subunit expressed alone in LLC-PK1 cells accumulates at the apical surface. When coexpressed with N519H, the H-K-ATPase beta -subunit assembles with this chimera and accompanies it to the basolateral surface. Thus the NH2-terminal basolateral signal in the Na-K-ATPase alpha -subunit masks or is dominant over any apical sorting information present in the beta -polypeptide. In gastric parietal cells, the H-K-ATPase beta -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-K-ATPase beta -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 beta -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.

cell polarity; protein chimera; ion pump; LLC-PK1 cells


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