Thermotropic behavior of retinal rod membranes and dispersions of extracted phospholipids

Abstract

High sensitivity, differential scanning calorimetry studies of bovine retinal rod outer segment (ROS) disk membranes and aqueous dispersions of the extracted ROS phospholipids have been performed. ROS disk membranes were found to exhibit a broad peak of excess heat capacity with a maximum at less than about 3 degrees C, ascribable to a gel-to-liquid crystalline phase transition of a fraction of the phospholipids. A similar thermotropic transition was observed for aqueous dispersions of the total extracted and purified ROS phospholipids. Comparison of the results obtained for the dispersion of total ROS phospholipids to those of the purified head group fractions suggests that the thermotropic behavior reflects a gel-to-liquid crystalline transition, leading to lateral phase separation, involving those phosphatidylcholine (PC) molecules containing saturated fatty acyl chains, possibly together with the highest melting ROS phosphatidylethanolamine (PE) and phosphatidylserine (PS) components. The interpretation of the thermal behavior of the ROS disk membranes depends on whether the transition is assumed to derive from the ROS PC and/or PE/PS fractions, and whether the transbilayer arrangement of the ROS phospholipids is assumed to be symmetric or asymmetric. The calorimetric data can be simply explained in terms of an asymmetric distribution of the major ROS disk membrane phospholipids (G.P. Miljanich et al., J. Membrane Biol. 60:249-255, 1981). In this case, the transition would arise from the PE/PS fractions in the outer ROS disk membrane monolayer, and the anticipated transition from the PC in the inner monolayer would be broadened due to interaction with cholesterol. For the ROS membranes at higher temperatures, two additional, irreversible transitions are observed at 57 and 72 degrees C, corresponding to the thermal denaturation of opsin and rhodopsin, respectively.

Effect of the C‐terminal proline repeats on ordered packing of squid rhodopsin and its mobility in membranes

“…The main parameter determining the derived rotational correlation time is the height ratio of the low field peaks (the two peaks on the left of the trace) at 15°C. branes show an irreversible transition at 65°C, which is not affected by removal of the C-terminus, and is similar to that seen for bovine rhodopsin [12][13][14].…”

“…The percentage of ordered regions for truncated rhodopsin membranes is much higher; a conservative lower limit for the percentage of lattice regions is 30%. branes show an irreversible transition at 65°C, which is not affected by removal of the C-terminus, and is similar to that seen for bovine rhodopsin [12][13][14]. cated a high degree of immobilization of the spin label with very little contamination of the spectrum from labelling of highly mobile groups.…”

Kinetic, Energetic, and Mechanical Differences between Dark-State Rhodopsin and Opsin

“…On average, the unfolding free energy of a stable structural segment lowered by 3.6 k B T. The thermal stability of rhodopsin and opsin has previously been studied by differential scanning calorimetry. The denaturation temperature was found to be 15°C lower for opsin (Albert et al, 1996; Khan et al, 1991; Miljanich et al, 1985; Shnyrov and Berman, 1988). This thermodynamic observation is complementary to our finding that opsin is energetically less stable.…”

Curvature Forces in Membrane Lipid–Protein Interactions

“…Rather, it is the transition from the lamellar phase to the reverse hexagonal ( H II ) (or cubic) phase of membrane phospholipids that is most affected by the proteolipid coupling (56–60). As one example the native retinal rod membranes contain bilayer-forming and nonbilayer-forming lipids (56), yet they are entirely in the fluid, liquid-crystalline ( L α ) state (also known as liquid-disordered or l d phase) near physiological temperature (56, 61). The tendency of lipids to bend is frustrated by the stretching energy of the acyl chains, or equivalently the solvation energy of the proteolipid interface—there is a balance of opposing contributions.…”