Ca2+ translocation across sarcoplasmic reticulum ATPase randomizes the two transported ions.
Cytoplasmic Ca2+ dissociation is sequential, and the Ca2+ ions bound to the nonphosphorylated ATPase are commonly represented as superimposed on each other, so that the superficial Ca2+ is freely exchangeable from the cytoplasm, whereas the deeper Ca2+ is not. Under conditions where ADP-sensitive phosphoenzyme accumulates (leaky vesicles, 5 degrees C, pH 8, 300 mM K+), luminal Ca2+ dissociation is sequential as well, so that the representation of two superimposed Ca2+ ions still holds on the phosphoenzyme, with the superficial Ca2+ facing the lumen freely exchangeable and the deeper Ca2+ blocked by the superficial Ca2+. Under the same conditions, we have investigated whether a prebuilt Ca2+ order is maintained during membrane translocation. Starting from a prebuilt order on the cytoplasmic side, we showed that the Ca2+ ions cannot be identified after translocation to the luminal side. The same result was obtained starting from a prebuilt order on the luminal side and following the luminal to cytoplasmic translocation. We conclude that the two Ca2+ ions are mixed during ATP-induced phosphorylation as well as during ADP-induced dephosphorylation.
Canet
D
D
Commissariat à l'Energie Atomique and Unité de Recherche 2096 Associée au Centre National de la Recherche Scientifique, Section de Biophysique des Protéines et des Membranes, Département de Biologie Cellulaire et Moléculaire, Centre d'
Forge
V
V
Guillain
F
F
Mintz
E
E
D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium