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J Biol Chem, Vol. 274, Issue 17, 11604-11610, April 23, 1999

The -Subunits of Na⁺,K⁺-ATPase and Gastric H⁺,K⁺-ATPase Have a High Preference for Their Own -Subunit and Affect the K⁺ Affinity of These Enzymes

From the Department of Biochemistry, Institute of Cellular Signaling, University of Nijmegen, P. O. Box 9101, 6500 HB Nijmegen, The Netherlands

The - and -subunits of Na⁺,K⁺-ATPase and H⁺,K⁺-ATPase were expressed in Sf9 cells in different combinations. Immunoprecipitation of the -subunits resulted in coprecipitation of the accompanying -subunit independent of the type of -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 -subunit of H⁺,K⁺-ATPase (NaKHK) showed an ATPase activity, which was only 12 ± 4% of the activity of the Na⁺,K⁺-ATPase with its own -subunit. Likewise, the complementary hybrid ATPase with the catalytic subunit of H⁺,K⁺-ATPase and the -subunit of Na⁺,K⁺-ATPase (HKNaK) showed an ATPase activity which was 9 ± 2% of that of the recombinant H⁺,K⁺-ATPase. In addition, the apparent K⁺ affinity of hybrid NaKHK was decreased, while the apparent K⁺ affinity of the opposite hybrid HKNaK was increased. The hybrid NaKHK 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 and NaKHK of 8800 ± 310 min¹ and 4800 ± 160 min¹, respectively. Measurements of phosphorylation of the HKNaK and HK enzymes are consistent with a higher turnover of the former. These findings suggest a role of the -subunit in the catalytic turnover. In conclusion, although both Na⁺,K⁺-ATPase and H⁺,K⁺-ATPase have a high preference for their own -subunit, they can function with the -subunit of the other enzyme, in which case the K⁺ affinity and turnover number are modified.

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