The Na+,K+-ATPase and the sarcoplasmic/endoplasmic reticulum Ca2+- (SERCA-) ATPase belong to a family of P-type ATPases that undergo a cycle of conformational changes between the phosphorylated and dephosphorylated stages in an ion-specific manner. The ouabain-inhibitable Na+,K+-ATPase activity requires Na+ and K+. On the other hand, the Ca2+-dependent and thapsigargin-inhibitable activity of the SERCA-ATPase does not depend upon Na+ and K+ for its basal activity. However, the SERCA-ATPase and Ca2+- transport activities can be further activated either by K+ in a two-step fashion with high (ED50 ~ 20 mM) and low affinity (ED50 ~ 70 mM) or by Na+ in a one-step fashion with an ED50 value of ~50 mM. A chimera, in which the carboxy-terminal region (Leu861-COOH) of the Na+,K+-ATPase α1 subunit replaced the corresponding region (Ser830-COOH) of the SERCAI- ATPase, lacked the low-affinity K+ activation of the SERCA-ATPase but displayed a higher-affinity (ED50 < 10 mM) activation by K+, similar to that of the Na+,K+-ATPase, whereas activation by Na+ was not affected. The replacement of the large cytosolic loop (Gly354-Lys712) and the amino- terminal regions (Met1-Asp162) of the SERCA1-ATPase with the corresponding portions of the Na+,K+-ATPase α1 subunit did not affect the sensitivity of the SERCA-ATPase activity to K+. Thus, the carboxy-terminal regions of both the SERCA1 and the Na+,K+-ATPase α1 subunit are critical for K+ sensitivity. Analysis of additional (Ca2+/Na+,K+)-ATPase chimeras demonstrated that the carboxy-terminal 102 amino acids (Phe920-Tyr1021) of the Na+/K+-ATPase α1 subunit are sufficient to shift the K+ affinity for activation of the SERCA-ATPase without the β subunit. No change in the two- step activation of SERCA-ATPase by K+ was seen when residues Thr871-Thr898 of the SERCA1-ATPase were replaced with residues Asn894-Ala919 of the Na+,K+-ATPase α1 subunit, a region known to bind the Na+,K+-ATPase β subunit [Lemas, M. V., et al. (1994) J. Biol. Chem. 269, 8255-8259]. Thus, the Na+,K+-ATPase subunit assembly domain and the K+-sensitive region are distinct within the carboxy-terminal 161 amino acids of the Na+,K+-ATPase.
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