Unusual partitioning behavior of CaATPase in dipalmitoylphosphatidylethanolamine/dielaidoylphosphatidylcholine++ + mixtures

Abstract

CaATPase from rabbit sarcoplasmic reticulum has been isolated, purified, stripped of its native lipids, and reconstituted into binary lipid mixtures of dielaidoylphosphatidylcholine (DEPC) and dipalmitoylphosphatidylethanolamine (DPPE) or acyl-chain perdeuterated DPPE (DPPE-d62). The partitioning properties of the protein were determined from differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy. Acyl-chain perdeuteration allows the separate determination of the order and melting characteristics of each lipid species with FT-IR. The binary lipid mixture has been shown to be phase separated in the gel state (Brauner, J. W., and R. Mendelsohn, 1986, Biochim. Biophys. Acta, 861:16-24). The solid phases present at low temperatures correspond to a pure DEPC phase and a mixed phase of DEPC/DPPE-d62. Insertion of protein at 37 degrees C leads to a domain of relatively protein-free DPPE-d62 and a phase containing both lipids plus CaATPase. We suggest that CaATPase selects a fixed composition (60% DEPC, 40% DPPE-d62) for its immediate environment. The composition of the lipid in the immediate vicinity of protein is largely independent of the initial DEPC/DPPE-d62 ratios in the reconstitution protocol. The relevance of these results to observations of discrete domains in native membranes is discussed.