Fig. (2) Characterization of the open membrane preparation. (A) SDS gel electrophoresis of SR vesicles and the membrane preparation obtained therefrom; lane a: marker enzymes, lane b: untreated SR vesicles, lane c: membrane fragments. The SDS treated membranes exhibit no significant change in the protein composition with respect to the SR vesicles. The prominent protein is the SR Ca-ATPase with a molecular mass of about 100 kDa. (The mass calculated from the 997 amino acids is 109.763 Da.) Electron-microscopic images of (B) SR vesicles and (C) open membranes obtained with negative stain technique. While the SR vesicles show globular structures with typical diameters of 100 – 200 nm, the open membranes showed irregularly shaped patches with diameter between 50 nm and 200 nm.