Fourier-transform infrared studies of calcium-ATPase partitioning in phospholipid mixtures of 1,2-dipalmitoylphosphatidylcholine-d62 with 1-palmitoyl-2-oleoylphosphatidylethanolamine and 1-stearoyl-2-oleoylphosphatidylcholine

Publisher Website
Google Scholar
Abstract
CaATPase from rabbit sarcoplasmic reticulum was reconstituted into binary lipid mixtures of 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE)/1,2-dipalmitoylphosphatidylcholine-d62(DPPC-d62) and 1-stearoyl-2-oloylphosphatidylcholine (SOPC)/DPPC-d62. Fourier-transform infrared (FT-IR) spectroscopy was used to monitor temperature-induced structural alterations in the individual lipid components in the presence and absence of protein. A simple 2-state model is used to construct a phase diagram that is in good agreement with 1 constructed from differential scanning calorimetry data, for the POPE/DPPC-d62 (protein-free) system. Although these 2 lipids are miscible over at least most of the composition range, substantial deviations from ideal behavior are observed. An estimate of the nonideality of mixing in both the gel and liquid-crystalline phases in obtained from regular solution theory. The phase diagram for SOPC/DPPC-d62 shows gel-phase immiscibility. FT-IR studies of ternary (POPE/DPPC-d62/CaATPase) complexes indicate that both lipid components are disordered by protein at all temperatures studied. In addition, their melting events are broadened and shifted to lower temperatures compared with the appropriate binary lipid mixture. Semiquantitative estimates for the fraction of each lipid melted are obtained from the model. The effect of protein of SOPC/DPPC-d62 mixtures depends on the total lipid to protein ratio. At low protein levels, SOPC is preferentially selected by CaATPase, so that bulk lipid is enriched in DPPC-d62. At high levels of protein, both lipid components are selected. The applicability of vibrational spectroscopy for determination of the partitioning preferences of membrane proteins into regions of particular chemical structure or physical order in a complex lipid environment is demonstrated.