Mapping a kingdom-specific functional domain of squalene synthase

• Published in: Biochimica et biophysica acta 2016-09, Vol.1861 (9), p.1049-1057
• Main Authors: Linscott, Kristin B., Niehaus, Thomas D., Zhuang, Xun, Bell, Stephen A., Chappell, Joe
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence - genetics; Ergosterol - metabolism; Farnesyl-Diphosphate Farnesyltransferase - genetics; Farnesyl-Diphosphate Farnesyltransferase - metabolism; Gene Knockout Techniques; Genetic complementation; Kingdom-of-life specificity; Mutation; Saccharomyces cerevisiae - enzymology; Saccharomyces cerevisiae - genetics; Squalene - metabolism; Squalene synthase; Sterol biosynthesis
• ISSN: 1388-1981; 0006-3002; 1879-2618
• DOI: 10.1016/j.bbalip.2016.06.008

Squalene synthase catalyzes the first committed step in sterol biosynthesis and consists of both an amino-terminal catalytic domain and a carboxy-terminal domain tethering the enzyme to the ER membrane. While the overall architecture of this enzyme is identical in eukaryotes, it was previously shown that plant and animal genes cannot complement a squalene synthase knockout mutation in yeast unless the carboxy-terminal domain is swapped for one of fungal origin. This implied a unique component of the fungal carboxy-terminal domain was responsible for the complementation phenotype. To identify this motif, we used Saccharomyces cerevisiae with a squalene synthase knockout mutation, and expressed intact and chimeric squalene synthases originating from fungi, plants, and animals. In contrast to previous observations, all enzymes tested could partially complement the knockout mutation when the genes were weakly expressed. However, when highly expressed, non-fungal squalene synthases could not complement the yeast mutation and instead led to the accumulation of a toxic intermediate(s) as defined by mutations of genes downstream in the ergosterol pathway. Restoration of the complete complementation phenotype was mapped to a 26-amino acid hinge region linking the catalytic and membrane-spanning domains specific to fungal squalene synthases. Over-expression of the C-terminal domain containing a hinge domain from fungi, not from animals or plants, led to growth inhibition of wild-type yeast. Because this hinge region is unique to and highly conserved within each kingdom of life, the data suggests that the hinge domain plays an essential functional role, such as assembly of ergosterol multi-enzyme complexes in fungi. •Non-fungal squalene synthase genes cannot complement the knockout mutation in yeast because a toxic intermediate accumulates•A fungal hinge domain is necessary and sufficient foreach squalene synthase gene to complement the knockout mutation•The 26-amino acid hinge motif is highly conserved and unique to each kingdom of life•Over-expression of any fungal hinge domain, but not that from plants or animals, inhibits the growth of wild-type yeast