Bacterial Analogs to Cholesterol Affect Dimerization of Proteorhodopsin and Modulates Preferred Dimer Interface.

Hopanoids, the bacterial analogues of sterols, are ubiquitous in bacteria and play a significant role in organismal survival under stressful environments. Unlike sterols, hopanoids have a high degree of variation in the size and chemical nature of the substituent attached to the ring moiety, leading to different effects on the structure and dynamics of biological membranes. While it is understood that hopanoids can indirectly tune membrane physical properties, little is known on the role that hopanoids may play in affecting the organization and behavior of bacterial membrane proteins. In this work we used coarse-grained molecular dynamics simulations to characterize the effects of two hopanoids, diploptene (DPT) and bacteriohopanetetrol (BHT), on the oligomerization of proteorhodopsin (PR) in a model membrane composed of 1-palmitoyl-2-oleoyl--glycero-3-phophoethanolamine (POPE) and 1-palmitoyl-2-oleoyl--3-phosphoglycerol (POPG). PR is a bacterial membrane protein that functions as a light-activated proton pump. We chose PR based on its ability to adopt a distribution of oligomeric states in different membrane environments. Furthermore, the efficiency of proton pumping in PR is intimately linked to its organization into oligomers. Our results reveal that both BHT and DPT indirectly affect dimerization by tuning membrane properties in a fashion that is concentration-dependent. Variation in their interaction with PR in the membrane-embedded and the cytoplasmic regions leads to distinctly different effects on the plasticity of the dimer interface. BHT has the ability to intercalate between monomers in the dimeric interface, whereas DPT shifts dimerization interactions via packing of the interleaflet region of the membrane. Our results show a direct relationship between hopanoid structure and lateral organization of PR, providing a first glimpse at how these bacterial analogues to eukaryotic sterols produce very similar biophysical effects within the cell membrane.