BIOSYNTHESIS OF TETRAETHER MEMBRANE LIPIDS IN THE ARCHAEON SULFOLOBUS ACIDOCALDARIUS (Invited Presentation)

Archaeal membrane lipids are distinct from the lipids observed in both bacteria and eukaryote. While bacterial and eukaryotic membranes are composed of fatty acids ester linked to glycerol-3-phosphate, archaeal membranes are composed of isoprenoidal chains ether bonded to glycerol-1-phosphate. In addition, many archaea can fuse their diether lipids to generate monolayer membranes known as glycerol dialkyl glycerol tetraethers (GDGTs) which can be further modified through the addition of pentacyclic rings. These unique structures are thought to provide a protective effect against the harsh environmental conditions encountered by many archaea. Archaeal lipids are also extremely useful as molecular fossils and paleotemperature proxies that have provided insight into the biogeochemical roles of archaea in marine environments and predictions of atmospheric and sea-surface temperatures throughout Earth history.

Although the uniqueness and utility of archaeal lipids has been well-established for many years, several questions remain regarding the biosynthesis and physiology of these lipids. In this study, we take a comparative genomics approach coupled with gene deletion analyses to address some of these unanswered questions in the thermoacidophilic archaeon Sulfolobus acidocaldarius. First, we identify a gene encoding a radical S-adenosylmethionine (SAM) protein, calditol synthase (Cds), necessary for the synthesis of calditol containing tetraether lipids known as glycerol dialkyl calditol tetraethers (GDCTs). Calditol is a pentacarbocycle that is ether linked to the glycerol backbone of GDGTs. Deletion of the Cds in S. acidocaldarius results in the complete loss of calditol production. Further, a calditol synthase mutant is unable to grow at extremely low pH indicating that calditol plays a functional role in protection against acidic stress. We also identify two distinct radical SAM proteins necessary for the introduction of the pentacyclic rings in the GDGT core. Deletion of these enzymes results in the production of GDGT lipids with no rings and results in reduced growth of S. acidocaldarius at higher temperatures. Taken together, these studies begin to provide answers to some of the more perplexing enzymatic and physiological questions regarding archaeal membranes.