Exoproteome analysis of Starmerella bombicola results in the discovery of an esterase required for lactonization of sophorolipids.

The yeast Starmerella bombicola secretes sophorolipids, a family of biosurfactants that find applications in green household products and cosmetics. Over the past years, a gene cluster was discovered that is responsible for the entire synthesis of the open (acidic) form of these molecules from glucose, fatty acids and acetyl-CoA building blocks. However, a significant fraction of the natural product is obtained as a ring closed form (lactonic). Both genetic and proteomic approaches hitherto failed to discover an enzyme responsible for the esterification reaction required for the ring closure step. We hypothesized that this enzyme is extracellularly secreted. Therefore, we characterized the composition of the S. bombicola exoproteome at different time points of the growth and compared it with known yeast exoproteomes. We identified 44 proteins, many of them commonly found in other fungi. Curiously, we discovered an enzyme with homology to Pseudozyma antarctica lipase A. A deletion mutation of its gene resulted in complete abolishment of the sophorolipid lactonization providing evidence that this might be the missing enzyme in the sophorolipid biosynthetic pathway. BIOLOGICAL SIGNIFICANCE: Growing concern about the impact of chemical processes on the environment increases consumers' demand for bio-based products. Lately, the household care and personal care sectors show increasing interest in naturally occurring biosurfactants, which constitute environment-friendly alternatives for chemical surfactants, typically derived from mineral oils. A particular group of biosurfactants, sophorolipids, already found their way to the market, being used in a range of household detergent products and in cosmetics. This work describes how proteomic approaches have led to the completion of our knowledge on the biosynthetic pathway of sophorolipids as performed by Starmerella bombicola, a fungus used in the industrial production of these biosurfactants. Moreover, we proved that by creating a deletion mutant in the lactone esterase discovered in this study, we can shape the biosynthesis towards custom-made sophorolipids with desired functions. Herewith, we demonstrate the potential of proteomics in industrial biotechnology.