Dimerization Determines Substrate Specificity of a Bacterial Prenyltransferase

The catalytic and structural basis of archaeal glycerophospholipid biosynthesis

Archaeal glycerophospholipids are the main constituents of the cytoplasmic membrane in the archaeal domain of life and fundamentally differ in chemical composition compared to bacterial phospholipids. They consist of isoprenyl chains ether-bonded to glycerol-1-phosphate. In contrast, bacterial glycerophospholipids are composed of fatty acyl chains ester-bonded to glycerol-3-phosphate. This largely domain-distinguishing feature has been termed the “lipid-divide”. The chemical composition of archaeal membranes contributes to the ability of archaea to survive and thrive in extreme environments. However, ether-bonded glycerophospholipids are not only limited to extremophiles and found also in mesophilic archaea. Resolving the structural basis of glycerophospholipid biosynthesis is a key objective to provide insights in the early evolution of membrane formation and to deepen our understanding of the molecular basis of extremophilicity. Many of the glycerophospholipid enzymes are either integral membrane proteins or membrane-associated, and hence are intrinsically difficult to study structurally. However, in recent years, the crystal structures of several key enzymes have been solved, while unresolved enzymatic steps in the archaeal glycerophospholipid biosynthetic pathway have been clarified providing further insights in the lipid-divide and the evolution of early life.

Quaternary Structure of the Tryptophan Synthase α-Subunit Homolog BX1 from Zea mays

BX1 from Zea mays (zmBX1) is an enzyme of plant secondary metabolism that generates indole for the synthesis of plant defensins. It is a homologue of the tryptophan synthase α-subunit, TrpA. Whereas TrpA itself is a monomer in solution, zmBX1 is dimeric, confirmed in our work by native MS. Using cross-linking and mutagenesis, we identified the physiological dimerization interface of zmBX1. We found that homodimerization has only minor effects on catalysis and stability. A comparison of the zmBX1-zmBX1 homodimer and zmTrpA-zmTrpB heterodimer interfaces suggest that homodimerization in zmBX1 might, at an early point in evolution, have served as a mechanism to exclude the interaction with the tryptophan synthase β-subunit (zmTrpB), marking its transition from primary to secondary metabolism.

Life before LUCA

We see the last universal common ancestor of all living organisms, or LUCA, at the evolutionary separation of the Archaea from the Eubacteria, and before the symbiotic event believed to have led to the Eukarya. LUCA is often implicitly taken to be close to the origin of life, and sometimes this is even stated explicitly. However, LUCA already had the capacity to code for many proteins, and had some of the same bioenergetic capacities as modern organisms. An organism at the origin of life must have been vastly simpler, and this invites the question of how to define a living organism. Even if acceptance of the giant viruses as living organisms forces the definition of LUCA to be revised, it will not alter the essential point that LUCA should be regarded as a recent player in the evolution of life.

Identification and Characterization of Heptaprenylglyceryl Phosphate Processing Enzymes in Bacillus subtilis

In Archaea, ether lipids play an essential role as the main building blocks of the cellular membrane. Recently, ether lipids have also been discovered in the domain of Bacteria, and the key enzymes that catalyze their synthesis, glycerol-1-phosphate dehydrogenase and heptaprenylglyceryl phosphate synthase, have been described. In Bacillales, heptaprenylglyceryl phosphate does not become linked to a second polyprenyl moiety like ether lipids in Archaea but is dephosphorylated and acetylated. Here, we report on the enzymes that catalyze these reactions. We enriched the phosphatase activity from a B. subtilis cell extract and suppose that dephosphorylation is catalyzed by the phosphatase PhoB or by any other phosphatase in an unspecific manner. By screening a B. subtilis knock-out library for deficiency in acetylation, the yvoF gene product was identified to be the acetyltransferase. The acetyl-CoA-dependent enzyme YvoF is a close relative of maltose O-acetyltransferase (MAT). Its catalytic properties were analyzed and compared with MAT. YvoF and MAT partially overlap in substrate and product range in vitro, but MAT is not able to complement the yvoF knock-out in vivo.

A comprehensive analysis of the geranylgeranylglyceryl phosphate synthase enzyme family identifies novel members and reveals mechanisms of substrate specificity and quaternary structure organization

Geranylgeranylglyceryl phosphate synthase (GGGPS) family enzymes catalyse the formation of an ether bond between glycerol-1-phosphate and polyprenyl diphosphates. They are essential for the biosynthesis of archaeal membrane lipids, but also occur in bacterial species, albeit with unknown physiological function. It has been known that there exist two phylogenetic groups (I and II) of GGGPS family enzymes, but a comprehensive study has been missing. We therefore visualized the variability within the family by applying a sequence similarity network, and biochemically characterized 17 representative GGGPS family enzymes regarding their catalytic activities and substrate specificities. Moreover, we present the first crystal structures of group II archaeal and bacterial enzymes. Our analysis revealed that the previously uncharacterized bacterial enzymes from group II have GGGPS activity like the archaeal enzymes and differ from the bacterial group I enzymes that are heptaprenylglyceryl phosphate synthases. The length of the isoprenoid substrate is determined in group II GGGPS enzymes by 'limiter residues' that are different from those in group I enzymes, as shown by site-directed mutagenesis. Most of the group II enzymes form hexamers. We could disrupt these hexamers to stable and catalytically active dimers by mutating a single amino acid that acts as an 'aromatic anchor'.