State of aggregation of the (Ca2+ + Mg2+)-ATPase studied using chemical cross-linking

We have studied cross-linking of the (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum and in reconstituted systems, using glutaraldehyde, cupric-1,10-phenanthroline and 3,3'-dithiobis (sulphosuccinimidylpropionate). All reagents produce extensive cross-linking, forming aggregates too large to enter polyacrylamide gels. Only traces of cross-linked dimeric ATPase species are formed. Saturation transfer electron spin resonance spectra of spin-labelled sarcoplasmic reticulum cross-linked with glutaraldehyde are also consistent with the formation of extensively cross-linked aggregates in the membrane. The results are interpreted in terms of dynamic clusters of ATPase molecules in the membrane, probably in the form of rows of ATPase molecules.