Overlapping Effects of S3 Stalk Segment Mutations on the Affinity of Ca2+-ATPase (SERCA) for Thapsigargin and Cyclopiazonic Acid

Publisher Website
Google Scholar
Abstract
Chimeric exchanges and mutations were produced in the Ca2+-ATPase (SERCA) to match (in the majority of cases) corresponding sequences of the Na+,K+-ATPase. The effects of these mutations on the concentration dependence of the specific Ca2+-ATPase inhibition by thapsigargin (TG) and cyclopiazonic acid (CPA) were then determined. Extensive chimeric mutations on the large cytosolic loop, on the S4 stalk segment, and on the M3 transmembrane segments produced little or no modification of the Ca2+-ATPase sensitivity to either inhibitor. On the other hand, the presence of a six amino acid Na+,K+-ATPase sequence within the S3 stalk segment of the Ca2+-ATPase raised 60-fold the apparent Ki for TG and 250-fold the apparent Ki for CPA. More limited mutations within the same S3 segment, however, affected differently the concentration dependence of the Ca2+-ATPase inhibition by TG or CPA. Specifically, single mutation of Phe256 to Val increased 20-fold the apparent Ki for TG, while having very little effect on the apparent Ki for CPA. These findings indicate significant overlap of the TG and CPA binding domains within the S3 stalk segment of the Ca2+-ATPase, where the contribution of each protein residue is dependent on the structures of the two inhibitors. Saturating concentrations of either or both TG and CPA produce an identical reduction of the affinity of the ATPase for ATP, suggesting that only one inhibitor can bind at any time due to significant overlap of their binding domains. It is suggested that perturbations produced by binding of either inhibitor within the stalk segment interfere with the long-range functional linkage between ATP utilization in the ATPase cytosolic region and Ca2+ binding in the membrane-bound region.