Protective and Inhibitory Effects of Various Types of Amphipols on the Ca2+-ATPase from Sarcoplasmic Reticulum: A Comparative Study†

Amphipols are amphipathic polymers designed to replace or supplement detergents in membrane protein solution studies. Previous work has suggested both advantages and disadvantages to the use of a polyacrylate-based amphipol, A8−35, for studying the sarcoplasmic reticulum Ca2+-ATPase (SERCA1a). We investigated this issue further using a set of four amphipols with different chemical structures. Previous size exclusion chromatography experiments had shown that A8−35 and SERCA1a/A8−35 complexes aggregate under certain conditions. We show here that aggregation can be prevented by omitting calcium from buffers or by using a sulfonated version of A8−35. A8−35 had previously been shown to protect Ca2+-ATPase from irreversible denaturation, while inhibiting its activity in a reversible manner. We show here that the other three amphipols tested also display these properties and that all four amphipols slow down backward calcium dissociation from the nonphosphorylated solubilized enzyme, a priori an unrelated step. ...