RNA-dependent synthesis of ergosteryl-3β-O-glycine in Ascomycota expands the diversity of steryl-amino acids

A wide range of bacteria possess virulence factors such as aminoacyl-tRNA transferases (ATTs) that are capable of rerouting aminoacyl-transfer RNAs away from protein synthesis to conjugate amino acids onto glycerolipids. We recently showed that, although these pathways were thought to be restricted to bacteria, higher fungi also possess ergosteryl-3β-O-L-aspartate synthases (ErdSs), which transfer the L-Asp moiety of aspartyl-tRNA(Asp) onto the 3β-OH group of ergosterol (Erg), yielding ergosteryl-3β-O-L-aspartate (Erg-Asp). Here, we report the discovery, in fungi, of a second type of fungal sterol-specific ATTs, namely, ergosteryl-3β-O-glycine (Erg-Gly) synthase (ErgS). ErgS consists of a freestanding DUF2156 domain encoded by a gene distinct from and paralogous to that of ErdS. We show that the enzyme only uses Gly-tRNA(Gly) produced by an independent glycyl-tRNA synthetase (GlyRS) to transfer glycine onto the 3β-OH of Erg, producing Erg-Gly. Phylogenomics analysis also show that the Erg-Gly synthesis pathway exists only in Ascomycota, including species of biotechnological interest, and more importantly, in human pathogens, such as Aspergillus fumigatus. The discovery of a second type of Erg-aa not only expands the repertoire of this particular class of fungal lipids but suggests that Erg-aa synthases might constitute a genuine subfamily of lipid-modifying ATTs.