Involvement of the β-subunit in the potassium access channel of the sodium pump

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The sodium pump composed of the α and β subunits, couples the hydrolysis of ATP to the translocation of NaィイD1+ィエD1 and KィイD1+ィエD1 across the cell membrane. How does the pump perform this translocation of cations and what structural domains of the pump are involved in this process? In this study we obtained the following results regarding these two problems.1. The chimera (βLィイD21ィエD2HH) is constructed by replacing the first disulfide-bonded loop (LィイD21ィエD2) of the sodium pump by the corresponding loop of the gastric proton pump, resulting in the formation of a functionless complex with the α subunit of the sodium pump. The chimeras (βLィイD21ィエD2HN and βL1NH) are constructed by replacing each half of the LィイD21ィエD2 of βLィイD21ィエD2HH by the corresponding portion of the sodium pump β subunit, forming partially active complexes with the α subunit. The result suggests that the N-terminal and C-terminal halves of LィイD21ィエD2 of the sodium pump function independently of each other. The mutatio … More n of Phe148 to Arg in the βLィイD21ィエD2HH restored the activity to the same extent as βLィイD21ィエD2HN, indicating that Arg148 is a critical residue in the C-terminal half of LィイD21ィエD2 of the sodium pump. In the N-terminal half, PADY sequence seems to be important for the sodium pump.2. Glu residues in transmembrane segments (Glu-334, -959 and -960) of the α subunit of the sodium pomp have been discussed as possible candidates for the binding sites of the transported cations. We demonstrated that none of the three Glu residues is essential for cation coordination, but that Glu-334, and in part also Glu-960, seems to be involved in preserving the ouabain-resistant conformation of the sodium pump.3. Polytoxin converts the sodium pump into an ion channel. We studied the effect of truncation of the N-terminus of the sodium pump α subunit on channel properties and pump functions. The results suggest that the N-terminus is involved in a voltage-dependent gate of the channel structure inherent in the pump molecule. Less