SPHINGOLIPIDS BIOSYNTHESIS IN BACTERIA

GlycoSphingoLipids (GSL) are amphiphilic molecules present in the plasma membrane of all eukaryotic cells where they play an important role in membrane structure and permeability, cell-cell recognition and interaction, and in modulation of the immune response. These fundamental functions make GSL essential components of eukaryotic cells. Although regarded as limited to the eukaryotic kingdom, a growing number of bacteria are recognized for their ability to produce GSL. Strikingly, the Sphingomonadaceae family possesses an outer membrane layer composed entirely of GSL instead of the canonical Gram-negative outer membrane component Lipopolysaccharides (LPS). Although the presence of GSL in Sphingomonadaceae was established, little is known about their biosynthetic pathways and their physiological importance. The enzyme serine palmitoyltransferase (SPT) catalyzes the first and rate limiting step of the sphingolipids biosynthetic pathway - the condensation of an activated fatty acid thioester with the amino acid serine to produce 3-ketodihydrosphingosine and is the only GSL related enzyme identified in bacteria so far. Using directed enzyme evolution followed by rigorous biochemical characterization and determination of the atomic structure, we show for the first time the complementation of a yeast SPT^TS mutant with a bacterial enzyme. This complementation was achieved following the introduction of a single mutation to the active site of the enzyme. The result further supports the role bacterial SPT plays in the bacterial biosynthesis of sphingolipids and provides a new and unique system to explore the role of GSL in bacteria.