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Ca 2+  transport by sarcoplasmic reticulum Ca 2+ -ATPase involves ATP-dependent phosphorylation of a catalytic aspartic acid residue. The key process, luminal Ca 2+  release occurs upon phosphoenzyme isomerization, abbreviated as  E 1PCa 2  (reactive to ADP regenerating ATP and with two occluded Ca 2+  at transport sites) →  E 2P (insensitive to ADP and after Ca 2+  release). The isomerization involves gathering of cytoplasmic actuator and phosphorylation domains with second transmembrane helix (M2), and is epitomized by protection of a Leu 119 -proteinase K (prtK) cleavage site on M2. Ca 2+  binding to the luminal transport sites of  E 2P, producing  E 2PCa 2  before Ca 2+ -release exposes the prtK-site. Here we explore  E 2P structure to further elucidate luminal gating mechanism and effect of membrane perturbation. We find that ground state  E 2P becomes cleavable at Leu 119  in a non-solubilizing concentration of detergent C 12 E 8  at pH 7.4, indicating a shift towards a more  E 2PCa 2 -like state. Cleavage is accelerated by Mg 2+  binding to luminal transport sites and blocked by their protonation at pH 6.0. Results indicate that possible disruption of phospholipid-protein interactions strongly favors an  E 2P species with looser head domain interactions at M2 and responsive to specific ligand binding at the transport sites, likely an early flexible intermediate in the development towards ground state  E 2P.

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Membrane Perturbation of ADP-insensitive Phosphoenzyme of Ca2+-ATPase Modifies Gathering of Transmembrane Helix M2 with Cytoplasmic Domains and Luminal Gating