Dynamics and structural responses to cis-trans isomerization in bacterial lipid bilayers

Bacterial and eukaryotic cells must respond to a changing environment, and have multiple adaptive mechanisms to respond to environmental stresses. Exogenous stresses such as temperature fluctuations and osmotic pressure are known to influence cell membrane fluidity and gene expression. To maintain membrane homeostasis, gram-negative bacteria show a short-term membrane composition response to temperature changes. Specifically, these bacteria isomerize unsaturated fatty acid tails in their bilayers, switching unsaturation sites from the more common cis isomer to the trans isomer. Cis-trans isomerization in unsaturated fatty acids increases cell membrane rigidity, decreasing fluidity of the lipid acyl tails. These changes maintain membrane homeostasis, but the effect size is difficult to quantify in vivo. In this work, we explore the impact of fatty acid cis-trans isomerization on the properties and dynamics in a membrane model based on Pseudomonas putida using molecular dynamics (MD) simulation. In our hypothetical model, we convert between all-cis and all-trans membranes, and report on the variation in membrane properties under these conditions. In addition to changes in membrane thickness and lipid diffusion, we find that the unsaturation site for a cis fatty acid has a higher probability to come to the membrane surface than the equivalent trans fatty acid. The reduced availability of unsaturation sites on the membrane surface may have downstream implications for their accessibility to enzymatic attack, potentially influencing the activity of cis-trans isomerase and other peripheral membrane proteins that act on lipid unsaturations. Since cis-trans isomerization can occur rapidly without new lipid biosynthesis, natural selection has adopted cis-trans isomerization as one of many responses to environmental stress.