Interaction of amphipols with sarcoplasmic reticulum Ca2+-ATPase

Amphipols are short-chain amphipathic polymers designed to keep membrane pr oteins soluble in aqueous solutions. We have evaluated the effects of the i nteraction of amphipols with sarcoplasmic reticulum Ca2+- ATPase either in a membrane-bound or a soluble form. If the addition of amphipols to deterge nt-solubilized ATPase was followed by removal of detergent, soluble complex es formed, but these complexes retained poor ATPase activity, were not very stable upon long incubation periods, and at high concentrations they exper ienced aggregation. Nevertheless, adding excess detergent to diluted deterg ent-free ATPase-amphipol complexes incubated for short periods immediately restored full activity to these complexes, showing that amphipols had prote cted solubilized ATPase from the rapid and irreversible inactivation that o therwise follows detergent removal. Amphipols also protected solubilized AT Pase from the rapid and irreversible inactivation observed in detergent sol utions if the ATPase Ca2+ binding sites remain vacant. Moreover, in the pre sence of Ca2+, amphipol/detergent mixtures stabilized concentrated ATPase a gainst inactivation and aggregation, whether in the presence or absence of lipids, for much longer periods of time (days) than detergent alone. Our ob servations suggest that mixtures of amphipols and detergents are promising media for handling solubilized Ca2+-ATPase under conditions that would othe rwise lead to its irreversible denaturation and/or aggregation.