Primary sequence and functional expression of a novel ouabain-resistant Na,K-ATPase. The beta subunit modulates potassium activation of the Na,K-pump

Area of Science:

• Molecular Biology
• Biochemistry
• Physiology

Background:

• Ouabain is a cardiac glycoside that inhibits the Na,K-ATPase pump.
• Understanding ouabain resistance mechanisms is crucial for pharmacology and physiology.
• The Na,K-ATPase is a vital ion transporter involved in maintaining cellular homeostasis.

Purpose of the Study:

• To elucidate the molecular basis of ouabain resistance in the toad Bufo marinus.
• To identify and characterize the Na,K-ATPase alpha and beta subunits involved in this resistance.
• To investigate the functional roles of different beta subunit isoforms in conjunction with the alpha 1 subunit.

Main Methods:

• Cloning of Na,K-ATPase alpha and beta subunits from toad urinary bladder cells.
• Functional expression of toad Na,K-ATPase subunits in Xenopus laevis oocytes.
• Measurement of Na,K-pump currents and determination of kinetic parameters (Ki, K1/2) for ouabain and potassium.

Main Results:

• Identified alpha 1, beta 1, and beta 3 Na,K-ATPase isoforms in toad cells.
• The toad alpha 1 isoform possesses a unique N-terminal sequence distinct from ouabain-resistant rat isoforms.
• Coexpression of alpha 1 with either beta 1 or beta 3 resulted in ouabain resistance, with identical maximum pump currents and ouabain inhibition constants.
• Distinct potassium activation constants (K1/2) were observed for alpha 1 beta 1 and alpha 1 beta 3, indicating beta subunit influence on potassium sensitivity.

Conclusions:

• The toad alpha 1 Na,K-ATPase isoform is both necessary and sufficient for conferring ouabain resistance.
• Beta 1 and beta 3 subunits can functionally associate with the alpha 1 subunit without affecting the ouabain-resistant phenotype.
• Specific sequences within the beta subunit can modulate the activation of the Na,K-pump by extracellular potassium ions.