Membrane stress caused by octanoic acid in Saccharomyces cerevisiae

In order to compete with petroleum-based fuel and chemicals, engineering a robust biocatalyst that can convert renewable feedstocks into biorenewable chemicals, such as carboxylic acids, is increasingly important. However, product toxicity is often problematic. In this study, the toxicity of the carboxylic acids hexanoic, octanoic, and decanoic acid on Saccharomyces cerevisiae was investigated, with a focus on octanoic acid. These compounds are completely inhibitory at concentrations of magnitude 1 mM, and the toxicity increases as chain length increases and as media pH decreases. Transciptome analysis, reconstruction of gene regulatory network, and network component analysis suggested decreased membrane integrity during challenge with octanoic acid. This was confirmed by quantification of dose-dependent and chain length-dependent induction of membrane leakage, though membrane fluidity was not affected. This induction of membrane leakage could be significantly decreased by a period of pre-adaptation, and this pre-adaptation was accompanied by increased oleic acid content in the membrane, significantly increased production of saturated lipids relative to unsaturated lipids, and a significant increase in the average lipid chain length in the membrane. However, during adaptation cell surface hydrophobicity was not altered. The supplementation of oleic acid to the medium not only elevated the tolerance of yeast cells to octanoic acid but also attenuated the membrane leakiness. However, while attempts to mimic the oleic acid supplementation effects through expression of the Trichoplusia ni acyl-CoA Δ9 desaturase OLE1(TniNPVE desaturase) were able to increase the oleic acid content, the magnitude of the increase was not sufficient to reproduce the supplementation effect and increase octanoic acid tolerance. Similarly, introduction of cyclopropanated fatty acids through expression of the Escherichia coli cfa gene was not helpful for tolerance. Thus, we have provided quantitative evidence that carboxylic acids damage the yeast membrane and that manipulation of the lipid content of the membrane can increase tolerance, and possibly production, of these valuable products.