The β-subunits of Na+,K+-ATPase and gastric H+,K+-ATPase have a high preference for their own α-subunit and affect the K+ affinity of these enzymes

Abstract

The α- and β-subunits of Na+,K+-ATPase and H+,K+-ATPase were expressed in Sf9 cells in different combinations. Immunoprecipitation of the α-subunits resulted in coprecipitation of the accompanying β-subunit independent of the type of β-subunit. This indicates cross-assembly of the subunits of the different ATPases. The hybrid ATPase with the catalytic subunit of Na+,K+-ATPase and the β-subunit of H+,K+-ATPase (NaKαHKβ) showed an ATPase activity, which was only 12 ± 4% of the activity of the Na+,K+-ATPase with its own β-subunit. Likewise, the complementary hybrid ATPase with the catalytic subunit of H+,K+-ATPase and the β-subunit of Na+,K+-ATPase (HKαNaKβ) showed an ATPase activity which was 9 ± 2% of that of the recombinant H+,K+-ATPase. In addition, the apparent K+ affinity of hybrid NaKαHKβ was decreased, while the apparent K+ affinity of the opposite hybrid HKαNaKβ was increased. The hybrid NaKαHKβ could be phosphorylated by ATP to a level of 21 ± 7% of that of Na+,K+-ATPase. These values, together with the ATPase activity gave turnover numbers for NaKαβ and NaKαHKβ of 8800 ± 310 min-1 and 4800 ± 160 min-1, respectively. Measurements of phosphorylation of the HKαNaKβ and HKαβ enzymes are consistent with a higher turnover of the former. These findings suggest a role of the β-subunit in the catalytic turnover. In conclusion, although both Na+,K+-ATPase and H+,K+-ATPase have a high preference for their own β-subunit, they can function with the β-subunit of the other enzyme, in which case the K+ affinity and turnover number are modified.