Wide-Angle X-Ray Scattering Comparative Analysis of Haloarchaeal Purple and Claret Membranes

We present the results of the study of the structural distinctions between the purple membrane (PM) of Halobacterium salinarum and the claret membrane (CM) of Haloquadratum walsbyi using small-angle and wide-angle X-ray scattering. PM and CM are rhodopsin-rich membrane regions. They are crucial for light-driven energy processes in extreme halophilic archaea. While PM structure has been extensively characterized, the structure of CM remains less understood. According to our data, PM exhibits a well-defined hexagonal crystalline lattice with unit cell parameters of a = b ≈ 62 Å, which is consistent with published data. Conversely, CM showed unexpected diffraction patterns, best fitted by rhombohedral lattice with unit cell dimensions a = b = 27.9 Å; γ = 82.9°. But the presence of several unindexed peaks indicates the complexity of the CM sample. These structural differences are attributed to variations in native lipid and pigment compositions, specifically the presence of bacterioruberin in CM, as confirmed by UV-visible spectroscopy. Bacterioruberin, a hallmark of CM, contributes to its red-shifted absorption spectrum, indicative of its organized state within the membrane. However, denaturing 12% polyacrylamide gel electrophoresis (SDS-PAGE) and electron microscopy reveal potential contamination in CM samples, suggesting the need for improved purification protocols. The findings highlight significant structural disparities between PM and CM, with implications for understanding membrane organization. This research underscores the critical role of lipids and pigments in shaping the supramolecular architecture of archaeal membranes and provides a foundation for future studies into their functional adaptations.