Two tail-anchored protein variants, differing in transmembrane domain length and intracellular sorting, interact differently with lipids

Abstract

C-tail-anchored (TA) proteins often require a transmembrane domain of moderate hydrophobicity to maintain their endoplasmic reticulum residence, but the suggested role of protein-lipid interactions in this phenomenon has not been established. Here, we studied the interaction of TA proteins with lipids by differential scanning calorimetry by using a model system consisting of liposomes embedding either of two forms of cytochrome b(5): the endoplasmic reticulum-resident wild-type (b(5)wt) and a mutant thereof (b(5)ext), that contains five extra nonpolar amino acids in its transmembrane domain and, therefore, reaches the plasma membrane. The proteins were incorporated into liposomes of palmitoyl-oleyl-phosphatidylcholine (POPC) or POPC mixed with either distearoyl-phosphatidylserine (DSPS), palmitoyl-oleyl-phosphatidylserine (POPS), distearoyl-phosphatidylcholine (DSPC), or C16-ceramide (CER). POPC liposomes displayed a single thermotropic transition centered at -3.4 degrees C. When present, the second lipid formed a domain within the POPC bilayer, as indicated by the appearance of an additional peak. This peak was centered at temperatures close to 0 degrees C in the case of liposomes containing 10% CER, DSPS, and POPS and at 23 degrees C in the case of DSPC, likely reflecting a higher degree of molecular packing for DSPC domains. In DSPS/POPC, POPS/POPC, or CER/POPC, but not in DSPC/POPC liposomes, the insertion of b(5)wt increased, whereas b(5)ext decreased, the relative contribution to the total enthalpy of the higher temperature, phase-separated component. These results were confirmed with fluorescence measurements by using pyrene-labeled phospholipids. The dissimilar interaction with lipids of these two differently localized TA proteins could have implications for their intracellular sorting.