title: Effect of Ca2+ on the dimeric structure of scallop sarcoplasmic reticulum.
abstract: Scallop sarcoplasmic reticulum (SR), visualized in situ by freeze-fracture and deep-etching, is characterized by long tubes displaying crystalline arrays of Ca2+-ATPase dimer ribbons, resembling those observed in isolated SR vesicles. The orderly arrangement of the Ca2+-ATPase molecules is well preserved in muscle bundles permeabilized with saponin. Treatment with saponin, however, is not needed to isolate SR vesicles displaying a crystalline surface structure. Omission of ATP from the isolation procedure of SR vesicles does not alter the dimeric organization of the Ca2+-ATPase, although the overall appearance of the tubes seems to be affected: the edges of the vesicles are scalloped and the individual Ca2+-ATPase molecules are not clearly defined. The effect of Ca2+ on isolated scallop SR vesicles was investigated by correlating the enzymatic activity and calcium-binding properties of the Ca2+-ATPase with the surface structure of the vesicles, as revealed by electron microscopy. The dimeric organization of the membrane is preserved at Ca2+ concentrations where the Ca2+ binds to the high affinity sites (half-maximum saturation at pCa approximately 7.0 with a Hill coefficient of 2.1) and the Ca2+-ATPase is activated (half-maximum activation at pCa approximately 6.8 with a Hill coefficient of 1.84). Higher Ca2+ concentrations disrupt the crystalline surface array of the SR tubes, both in the presence and absence of ATP. We discuss here whether the Ca2+-ATPase dimer identified as a structural unit of the SR membrane represents the Ca2+ pump in the membrane.
authors.affiliation: Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110.
authors.forename: L
authors.initials: L
authors.lastname: Castellani
authors.forename: P M
authors.initials: PM
authors.lastname: Hardwicke
authors.forename: C
authors.initials: C
authors.lastname: Franzini-Armstrong
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D005613:Freeze Etching; D005614:Freeze Fracturing; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008974:Mollusca / Q000648:ultrastructure*; D009132:Muscles / Q000201:enzymology / Q000648:ultrastructure; D012519:Sarcoplasmic Reticulum / Q000201:enzymology* / Q000648:ultrastructure
chemical_list: D046911:Macromolecular Substances; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium