Effects of inhibitors on luminal opening of Ca2+ binding sites in an E2P-like complex of sarcoplasmic reticulum Ca22+-ATPase with Be22+-fluoride

We document here the intrinsic fluorescence and 45Ca2+ binding properties of putative "E2P-related" complexes of Ca2+-free ATPase with fluoride, formed in the presence of magnesium, aluminum, or beryllium. Intrinsic fluorescence measurements suggest that in the absence of inhibitors, the ATPase complex with beryllium fluoride (but not those with magnesium or aluminum fluoride) does constitute an appropriate analog of the "ADP-insensitive" phosphorylated form of Ca2+-ATPase, the so-called "E2P" state. 45Ca2+ binding measurements, performed in the presence of 100 mm KCl, 5 mm Mg2+, and 20% Me2SO at pH 8, demonstrate that this ATPase complex with beryllium fluoride (but again not those with magnesium or aluminum fluoride) has its Ca2+ binding sites accessible for rapid, low affinity (submillimolar) binding of Ca2+ from the luminal side of SR. In addition, we specifically demonstrate that in this E2P-like form of ATPase, the presence of thapsigargin, 2,5-di-tert-butyl-1,4-dihydroxybenzene, or cyclopiazonic acid prevents 45Ca2+ binding (i.e. presumably prevents opening of the 45Ca2+ binding sites on the SR luminal side). Since crystals of E2P-related forms of ATPase have up to now been described in the presence of thapsigargin only, these results suggest that crystallizing an inhibitor-free E2P-like form of ATPase (like its complex with beryllium fluoride) would be highly desirable, to unambiguously confirm previous predictions about the exit pathway from the ATPase transmembrane Ca2+ binding sites to the SR luminal medium.