Organization of calcium pump protein dimers in the isolated sarcoplasmic reticulum membrane.

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.