Ca2+ Release to Lumen from ADP-sensitive Phosphoenzyme E1PCa2 without Bound K+ of Sarcoplasmic Reticulum Ca2+-ATPase

During Ca2+ transport by sarcoplasmic reticulum Ca2+-ATPase, the conformation change of ADP-sensitive phosphoenzyme (E1PCa(2)) to ADP-insensitive phosphoenzyme (E2PCa(2)) is followed by rapid Ca2+ release into the lumen. Here, we find that in the absence of K+, Ca2+ release occurs considerably faster than E1PCa(2) to E2PCa(2) conformation change. Therefore, the lumenal Ca2+ release pathway is open to some extent in the K+-free E1PCa(2) structure. The Ca2+ affinity of this E1P is as high as that of the unphosphorylated ATPase (E1), indicating the Ca2+ binding sites are not disrupted. Thus, bound K+ stabilizes the E1PCa(2) structure with occluded Ca2+, keeping the Ca2+ pathway to the lumen closed. We found previously (Yamasaki, K., Wang, G., Daiho, T., Danko, S., and Suzuki, H. (2008) J. Biol. Chem. 283, 29144-29155) that the K+ bound in E2P reduces the Ca2+ affinity essential for achieving the high physiological Ca2+ gradient and to fully open the lumenal Ca2+ gate for rapid Ca2+ release (E2PCa(2)-> E2P + 2Ca(2+)). These findings show that bound K+ is critical for stabilizing both E1PCa(2) and E2P structures, thereby contributing to the structural changes that efficiently couple phosphoenzyme processing and Ca2+ handling.