title: Organization of calcium pump protein dimers in the isolated sarcoplasmic reticulum membrane.
abstract: The arrangement of the calcium pump protein in the isolated sarcoplasmic reticulum (SR) membrane was examined by optical diffraction of freeze-fracture electron micrographs. Several states of protein particle organization were observed: random, hexagonal and tetragonal packing, and a mixture of hexagonal and tetragonal packing. This suggests that the time-averaged positions of protein particles in the plane of the SR membrane are weakly defined. In addition, there appears to be a greater degree of local or short-range order compared to long-range order within the field of freeze-fracture particles. We utilized measurements from tetragonally or hexagonally packed arrays to determine a unit cell area occupied by each freeze-fracture particle and its associated lipid matrix. When these unit cell areas and the stereologically determined area per freeze-fracture particle were compared to the cross-sectional area occupied by a single calcium pump protein and its associated lipid, obtained by x-ray and neutron diffraction methods, we concluded that each freeze-fracture particle probably represents a dimer of pump protein molecules in the plane of the SR membrane.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D005614:Freeze Fracturing; D008433:Mathematics; D008854:Microscopy, Electron; D008961:Models, Structural; D009132:Muscles / Q000378:metabolism; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism / Q000648:ultrastructure*
chemical_list: D000252:Calcium-Transporting ATPases; D002118:Calcium