Varying the position of phospholipid acyl chain unsaturation modulates hopanoid and sterol ordering

The cell membrane acts as a responsive interface between the cell and its surroundings. Ultimately, the diverse panel of lipids comprising the lipidome are employed to tune membrane biophysical properties for optimal function. For eukaryotes, a family of amphiphiles called sterols are crucial due to their unique capacity to modulate the order of membrane lipids. Bacteria typically lack sterols, however, some can synthesize a family of sterol-analogues called hopanoids. Hopanoids are recognized as bacterial analogs of sterols due to their chemical and biophysical similarities. Notably, hopanoids are proposed as evolutionary sterol precursors since they have been found in ancient sediments and their biosynthesis could have proceeded prior to the oxygenation of Earths surface. While hopanoids and sterols can both impart order to saturated phospholipids, this interaction changes vastly with the presence of a double bond in the phospholipids acyl chain. Here, we present a study examining how the unsaturation position along the acyl chain influences the ordering effect of sterols (cholesterol) and hopanoids (diplopterol). We found that diplopterol and cholesterol exhibit different ordering effects on unsaturated lipids, depending on the relative positions of the double bond and the methyl groups of cholesterol/diplopterol. Moreover, in the bacterium Mesoplasma florum, diplopterol's interplay with unsaturated lipid isomers modulates bacterial membrane robustness. These results reveal how subtle changes in lipid structure can influence the membrane's collective properties and introduces double bond position as a modifiable lipidomic feature that cells can employ to fine-tune their membrane for adaptation to environmental change.