title: Characterization of the single Ca(2+)-binding site on the Ca(2+)-ATPase reconstituted with short- or long-chain phosphatidylcholines.
abstract: On reconstitution of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum into bilayers of dimyristoleoylphosphatidylcholine [di(C14:1)PC] or dinervonylphosphatidylcholine [di(C24:1)PC] the stoichiometry of Ca2+ binding changes from the usual two Ca2+ ions bound per ATPase molecule to one Ca2+ ion bound per ATPase molecule. For the ATPase in di(C24:1)PC, removal of Ca2+ from the Ca(2+)-bound ATPase results in a decrease in tryptophan fluorescence intensity, as observed for the ATPase in dioleoylphosphatidylcholine [di(C18:1)PC]. For the ATPase in di(C14:1)PC removal of Ca2+ results in no change in tryptophan fluorescence intensity. In the presence of Mg2+, removal of Ca2+ from the ATPase in di(C18:1)PC or di(C24:1)PC results in a decrease in tryptophan fluorescence intensity, but for the ATPase in di(C14:1)PC this results in an increase in intensity. Fluorescence of the ATPase labelled with 4-nitrobenzo-2-oxa-1,3-diazole (NBD) is the same for the ATPase in di(C18:1)PC or di(C24:1)PC, but is markedly greater in di(C14:1)PC, consistent with a 4-fold increase in the E1/E2 equilibrium constant. Addition of Mg2+ to NBD-labelled ATPase in di(C18:1) PC or di(C24:1)PC results in an increase in NBD fluorescence, attributed to stronger binding of Mg2+ to the E1 than to the E2 conformation; addition of Mg2+ had no effect on the fluorescence of the NBD-labelled ATPase in di(C14:1)PC. In the absence of Ca2+, Mg2+ increased the tryptophan fluorescence of the ATPase in di(C14:1)PC, di(C18:3)PC or di(C24:1)PC, with the same binding-constant for Mg2+ in all three lipids. Addition of Mg2+ to the ATPase labelled with 4-(bromomethyl)-6,7-dimethoxycoumarin resulted in a decrease in fluorescence in di(C18:1)PC or di(C24:1)PC but had no effect in di(C14:1)PC. These effects are interpreted in terms of binding of Ca2+ at a single outer Ca2+ binding-site on the ATPase in di(C14:1)PC and di(C24:1)PC, in a conformation in which the inner site is occluded [in di(C14:1)PC] or modified in its affinity for Ca2+ [in di(C24:1)PC]. Thapsigargin binds to the ATPase, reducing its affinity for Ca2+ both in di(C14:1)PC and di(C24:1)PC.
authors.affiliation: Department of Biochemistry, University of Southampton, UK.
authors.forename: A P
authors.initials: AP
authors.lastname: Starling
authors.forename: Y M
authors.initials: YM
authors.lastname: Khan
authors.forename: J M
authors.initials: JM
authors.lastname: East
authors.forename: A G
authors.initials: AG
authors.lastname: Lee
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000737:chemistry / Q000378:metabolism*; D003374:Coumarins; D005456:Fluorescent Dyes; D008274:Magnesium / Q000494:pharmacology; D018482:Muscle, Skeletal / Q000201:enzymology*; D010069:Oxadiazoles; D010713:Phosphatidylcholines / Q000378:metabolism*; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000737:chemistry
chemical_list: D003374:Coumarins; D005456:Fluorescent Dyes; D010069:Oxadiazoles; D010713:Phosphatidylcholines; C011514:4-nitrobenzofurazan; C053769:4-bromomethyl-6,7-dimethoxycoumarin; D014364:Tryptophan; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium