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

The alpha- and beta-subunits of Na+,K+-ATPase and H+,K+ ATPase were express ed in Sf9 cells in different combinations. Immunoprecipitation of the alpha -subunits resulted in coprecipitation of the accompanying beta-subunit inde pendent of the type of beta-subunit, This indicates cross-assembly of the s ubunits of the different ATPases, The hybrid ATPase with the catalytic subu nit of Na+,K+- ATPase and the beta-subunit of H+,K+-ATPase (NaK alpha HK be ta) showed an ATPase activity, which was only 12 +/- 4% of the activity of the Na+,K+-ATPase with its own beta-subunit. Likewise, the complementary hy brid ATPase with the catalytic subunit of H+,K+-ATPase and the beta-subunit of Na+,K+-ATPase (HK alpha NaK beta) showed an ATPase activity which was 9 +/- 2% of that of the recombinant H+,K+ ATPase, In addition, the apparent K+ affinity of hybrid NaK alpha HK beta was decreased, while the apparent K + affinity of the opposite hybrid HK alpha NaK beta was increased. The hybr id NaK alpha HK beta 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 NaK alpha beta and NaK alpha HK beta of 8800 +/- 310 min(-1) and 4800 +/- 160 min(-1), respectively. Measurements of phospho rylation of the HK alpha NaK beta and HK alpha beta enzymes are consistent with a higher turnover of the former. These findings suggest a role of the beta-subunit in the catalytic turnover. In conclusion, although both Na+,K-ATPase and H+,K+-ATPase have a high preference for their own beta-subunit, they can function with the beta-subunit of the other enzyme, in which case the K+ affinity and turnover number are modified.