Abstract
The sarcoplasmic reticulum Ca‐ATPase was subjected to target size analysis by radiation inactivation in various buffer conditions and after solubilization in monomeric form in non‐ionic detergent and in SDS. The target size was also determined for Ca‐ATPase in bidimensional crystals formed in the presence of decavanadate or lanthanide. The standardization obtained with defined monomers of Ca‐ATPase shows that the target size of Ca‐ATPase in the functional membrane‐bound state may be ascribed to a single peptide chain, possibly with surrounding lipid. Further analysis of the radiation inactivation sizes of various partial reactions of the pump cycle, including phosphorylation and Ca2+ occlusion, indicated much smaller values than the target size pertaining to decomposition of the whole peptide chain. This is consistent with the existence of separate functional domains within a single peptide chain.

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