Sorting of P-type ATPases in polarized epithelial cells

Area of Science:

• Cell Biology
• Molecular Biology
• Physiology

Background:

• Na,K-ATPase and H,K-ATPase are homologous P-type ion pumps with distinct cellular localizations.
• Na,K-ATPase is basolateral, while H,K-ATPase is apical in stomach parietal cells.
• Understanding these localization differences is key to regulating gastric acid secretion.

Purpose of the Study:

• To identify sequence domains responsible for H,K-ATPase targeting to the apical membrane.
• To elucidate the mechanism regulating H,K-ATPase internalization and gastric acid secretion cessation.

Main Methods:

• Construction and expression of molecular chimeras between Na,K-ATPase and H,K-ATPase alpha-subunits.
• Expression of pump constructs in polarized epithelial cells.
• Site-directed mutagenesis of the H,K-ATPase beta-subunit and generation of transgenic mice.

Main Results:

• The fourth transmembrane domain of H,K-ATPase contains apical targeting information.
• A tyrosine-based signal in the H,K-ATPase beta-subunit is essential for pump endocytosis.
• Transgenic mice with a mutated beta-subunit exhibit constitutive gastric acid hypersecretion due to continuous apical H,K-ATPase presence.

Conclusions:

• Apical localization of H,K-ATPase is determined by signals within its alpha-subunit (specifically, TM4).
• The H,K-ATPase beta-subunit contains a critical tyrosine-based signal for internalization, regulating acid secretion cessation.
• Proper targeting and regulation of the gastric H,K pump require at least two distinct sorting signals.