Search

FtsZ is a prokaryotic homologue of the eukaryotic cytoskeletal protein tubulin and plays a central role in prokaryotic cell division. Both FtsZ and tubulin are known to pass through cycles of polymerization and depolymerization, but the structural mechanisms underlying this cycle remain to be determined. Comparison of tubulin structures obtained in different states has led to a model in which the tubulin monomer undergoes a conformational switch between a "straight" form found in the walls of microtubules and a "curved" form associated with depolymerization, and it was proposed recently that this model may apply also to FtsZ. Here, we present new structures of FtsZ from47 Aquifex aeolicus,47 Bacillus subtilis, Methanococcus jannaschii and Pseudomonas aeruginosa that provide strong constraints on any proposed role for a conformational switch in the FtsZ monomer. By comparing the full range of FtsZ structures determined in different crystal forms and nucleotide states, and in the presence or in the absence of regulatory proteins, we find no evidence of a conformational change involving domain movement. Our new structural data make it clear that the previously proposed straight and curved conformations of FtsZ were related to inter-species differences in domain orientation rather than two interconvertible conformations. We propose a new model in which lateral interactions help determine the curvature of protofilaments.

The thermophilic chemolithotroph, Aquifex aeolicus, expresses a gene product that exhibits both ATP sulfurylase and adenosine-5'-phosphosulfate (APS) kinase activities. These enzymes are usually segregated on two separate proteins in most bacteria, fungi, and plants. The domain arrangement in the Aquifex enzyme is reminiscent of the fungal ATP sulfurylase, which contains a C-terminal domain that is homologous to APS kinase yet displays no kinase activity. Rather, in the fungal enzyme, the motif serves as a sulfurylase regulatory domain that binds the allosteric effector 3'-phosphoadenosine-5'-phosphosulfate (PAPS), the product of true APS kinase. Therefore, the Aquifex enzyme may represent an ancestral homolog of a primitive bifunctional enzyme, from which the fungal ATP sulfurylase may have evolved. In heterotrophic sulfur-assimilating organisms such as fungi, ATP sulfurylase catalyzes the first committed step in sulfate assimilation to produce APS, which is subsequently metabolized to generate all sulfur-containing biomolecules. In contrast, ATP sulfurylase in sulfur chemolithotrophs catalyzes the reverse reaction to produce ATP and sulfate from APS and pyrophosphate. Here, the 2.3 A resolution X-ray crystal structure of Aquifex ATP sulfurylase-APS kinase bifunctional enzyme is presented. The protein dimerizes through its APS kinase domain and contains ADP bound in all four active sites. Comparison of the Aquifex ATP sulfurylase active site with those from sulfate assimilators reveals similar dispositions of the bound nucleotide and nearby residues. This suggests that minor perturbations are responsible for optimizing the kinetic properties for the physiologically relevant direction. The APS kinase active-site lid adopts two distinct conformations, where one conformation is distorted by crystal contacts. Additionally, a disulfide bond is observed in one ATP-binding P-loop of the APS kinase active site. This linkage accounts for the low kinase activity of the enzyme under oxidizing conditions. The thermal stability of the Aquifex enzyme can be explained by the 43% decreased cavity volume found within the protein core.

The 2.3 A resolution crystal structure of a [2Fe-2S] cluster containing ferredoxin from Aquifex aeolicus reveals a thioredoxin-like fold that is novel among iron-sulfur proteins. The [2Fe-2S] cluster is located near the surface of the protein, at a site corresponding to that of the active-site disulfide bridge in thioredoxin. The four cysteine ligands are located near the ends of two surface loops. Two of these ligands can be substituted by non-native cysteine residues introduced throughout a stretch of the polypeptide chain that forms a protruding loop extending away from the cluster. The presence of homologs of this ferredoxin as components of more complex anaerobic and aerobic electron transfer systems indicates that this is a versatile fold for biological redox processes.