Roles of Tyr122-hydrophobic Cluster and K+ Binding in Ca2+-releasing Process of ADP-insensitive Phosphoenzyme of Sarcoplasmic Reticulum Ca2+-ATPase

Abstract

Tyr122 -hydrophobic cluster (Y122-HC) is an interaction network formed by the top part of the second transmembrane helix and the cytoplasmic actuator and phosphorylation domains of sarcoplasmic reticulum Ca 2؉ -ATPase. We have previously found that Y122-HC plays critical roles in the processing of ADP-insensitive phosphoenzyme (E2P) after its formation by the isomerization from ADP-sensitive phosphoenzyme (E1PCa 2 ) (Wang, G., Yamasaki, K., Daiho, T., and Suzuki, H. (2005) This EP is rapidly dephosphorylated by ADP in the reverse reaction reproducing ATP, therefore "ADP-sensitive EP" (E1P). In the next step (step 4), E1PCa 2 is isomerized to the ADP-insensitive form, E2PCa 2 . Upon this change at the catalytic site, the Ca 2ϩ sites are deoccluded and opened to the lumenal side, and the Ca 2ϩ affinity is largely reduced, releasing the bound Ca 2ϩ ions into the lumen (step 5). The Ca 2ϩ release process is thought to be very rapid with the wild-type Ca 2ϩ -ATPase, and the accumulation of E2PCa 2 intermediate had actually never been found until we recently identified and trapped successfully this intermediate by a mutation study (9). In the final step, the Asp 351 -acylphosphate of E2P is hydrolyzed to reproduce the dephosphorylated and inactive E2 form (step 6). The transport cycle is totally reversible, e.g. E2P can be formed from E2 by P i in the absence of Ca 2ϩ , and the subsequent lumenal Ca 2ϩ binding to E2P produces E1PCa 2 .Three-dimensional structures in several intermediate states and their analogs have been solved (10 -18). The Ca 2ϩ -ATPase has three cytoplasmic domains, P (phosphorylation), N (nucleotide binding), and A (actuator or anchor), and ten transmembrane helices (M1-M10). The two Ca 2ϩ binding sites consist of residues on M4, M5, M6, and M8 (10). The P domain possesses the phosphorylation site (Asp 351 ) and is directly linked to the long helices M4 and M5. The ATP binding site is on the N domain connected to the P domain. The A domain is linked to M1, M2, and M3 via the A/M1-, A/M2-, and A/M3-linkers. The cytoplasmic three domains largely move and change their organization states during the Ca 2ϩ -transport cycle (19 -21), and these changes are linked with the rearrangements in the transmembrane helices for the Ca 2ϩ transport. As a most remarkable change, in the EP isomerization (loss of ADP sensitivity) and Ca 2ϩ release, the A domain largely rotates and the P domain largely inclines toward the A domain, and these domains produce their tight association (see Fig. 1 for the change E1Ca 2 ⅐AlF 4 Ϫ ⅐ADP 3 E2⅐MgF 4 2Ϫ as the model for the overall process E1ϳPCa 2 ⅐ADP 3 E2⅐P i , including the EP isomerization and Ca 2ϩ release). These structural changes therefore involve distinct events in distinct regions, yet they * This work was supported by a grant-in-aid for scientific research (C) (to K. Y.) and (B) (to H. S.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The costs of publication of this article were defrayed in part by the payment of page charges. ...

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

“…E1PCa 2 to E2P transition. The result agrees with the established mechanism that the two Ca 2ϩ ions are occluded in E1PCa 2 , and Ca 2ϩ release into the lumen occurs very rapidly after the rate-limiting E1PCa 2 to E2PCa 2 transition, E1PCa 2 3 E2PCa 2 3 E2P ϩ 2Ca 2ϩ (11)(12)(13)(14). Thus, the EP transition and Ca 2ϩ release are tightly coupled in the presence of K ϩ .…”

“…Thus, in the natural E1PCa 2 structure, bound K ϩ stabilizes the Ca 2ϩ in an occluded form by not allowing the pathway to open. Bound K ϩ also stabilizes E2P following disruption of the Ca 2ϩ sites and full opening of the lumenal gate (13). Thus, bound K ϩ has a crucial role in EP processing and Ca 2ϩ occlusion and release to the lumen in the sequence E1PCa 2 3 E2PCa 2 3 E2P ϩ 2Ca 2ϩ .…”

“…Thus, although progression to sE1PCa 2 is relatively fast, this pathway cannot contribute to gradient formation and ultimately slows normal transport. It is concluded that K ϩ ensures the normal structural process for Ca 2ϩ transport (blue arrows) by stabilizing the Ca 2ϩ -occluded structure of E1PCa 2 and disallowing opening of a lumenal Ca 2ϩ pathway (this study), and by stabilizing the E2P structure with disrupted Ca 2ϩ sites (greatly reduced affinity) and a fully opened lumenal gate (13). on P␣5 of this region) and by the Glu 732 side chain oxygen on P␣7 (Fig.…”

“…In the P domain, there is a specific K ϩ binding site (28); K ϩ binding here is crucial for rapid hydrolysis of E2P (28 -30). Recently, we further found (13) that the K ϩ in E2P is critical for reducing the lumenal Ca 2ϩ affinity that is required to achieve the high physiological Ca 2ϩ gradient and for rapid Ca 2ϩ release (E2PCa 2 3 E2P ϩ 2Ca 2ϩ ). Thus, bound K ϩ contributes to stabilization of the compactly organized and inclined E2P structure with its disrupted Ca 2ϩ sites and fully opened lumenal gate, probably by cross-linking the P domain with the A domain/M3-linker (13).…”

“…Thus bound K ϩ likely cross-links the bottom part of the P domain and the A/M3-linker. This cross-link must contribute to the stabilization of the compactly organized and inclined E2P structure with disrupted Ca 2ϩ sites and fully opened lumenal gate (13).…”