Isolated sarcoplasmic reticulum (SR) vesicles with polymerized calcium pump protein were freeze-dried and rotary shadowed following uranyl acetate stabilization. This technique allows direct observation of a single side of the vesicle without requiring optical filtering. The heads of individual ATPase molecules, projecting above the cytoplasmic surface, are clearly resolved in the replicas. Ca ATPase molecules form extensive arrays in vanadate-treated, rabbit SR vesicles and in gently isolated, native SR vesicles from scallop. Gentle isolation results in limited areas of orderly structure in native SR isolated from vertebrate muscles. Special attention is given to the effect of various shadow thicknesses on the appearance of the heads. This information is essential to the interpretation of images in the accompanying paper (Franzini-Armstrong, C., and D.J. Ferguson, 1985, Biophys. J., 48:607–615).
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