Crystallization and preliminary crystallographic analysis of two Streptococcus agalactiae proteins: the family II inorganic pyrophosphatase and the serine/threonine phosphatase

Perturbation of manganese metabolism disrupts cell division in Streptococcus pneumoniae

Manganese (Mn) is an essential micronutrient and required cofactor in bacteria. Despite its importance, excess Mn can impair bacterial growth, the mechanism of which remains largely unexplored. Here, we show that proper Mn homeostasis is critical for cellular growth of the major human respiratory pathogen Streptococcus pneumoniae. Perturbations in Mn homeostasis genes, psaBCA, encoding the Mn importer, and mntE, encoding the Mn exporter, lead to Mn sensitivity during aerobiosis. Mn-stressed cells accumulate iron and copper, in addition to Mn. Impaired growth is a direct result of Mn toxicity and does not result from iron-mediated Fenton chemistry, since cells remain sensitive to Mn during anaerobiosis or when hydrogen peroxide biogenesis is significantly reduced. Mn-stressed cells are significantly elongated, whereas Mn-limitation imposed by zinc addition leads to cell shortening. We show that Mn accumulation promotes aberrant dephosphorylation of cell division proteins via hyperactivation of the Mn-dependent protein phosphatase PhpP, a key enzyme involved in the regulation of cell division. We discuss a mechanism by which cellular Mn:Zn ratios dictate PhpP specific activity thereby regulating pneumococcal cell division. We propose that Mn-metalloenzymes are particularly susceptible to hyperactivation or mismetallation, suggesting the need for exquisite cellular control of Mn-dependent metabolic processes.

Structure of the Streptococcus agalactiae family II inorganic pyrophosphatase at 2.80 A resolution

Streptococcus agalactiae, a prokaryote that causes infections in neonates and immunocompromised adults, has a serine/threonine protein kinase (STK) signalling cascade. The structure of one of the targets, a family II inorganic pyrophosphatase, has been solved by molecular replacement and refined at 2.80 A resolution to an R factor of 19.2% (R(free) = 26.7%). The two monomers in the asymmetric unit are related by a noncrystallographic twofold axis, but the biological dimer is formed by a crystallographic twofold. Each monomer contains the pyrophosphate analogue imidodiphosphate (PNP) and three metal ions per active site: two Mn(2+) ions in sites M1 and M2 and an Mg(2+) ion in site M3. The enzyme is in the closed conformation. Like other family II enzymes, the structure consists of two domains (residues 1-191 and 198-311), with the active site located between them. The conformation of Lys298 in the active site is different from those observed previously and it coordinates to the conserved DHH motif in a unique way. The structure suggests that Ser150, Ser194, Ser195 and Ser296 are the most likely targets for the Ser/Thr kinase and phosphatase because they are surface-accessible and either in the active site or in the hinge region between the two domains.

Structure of Streptococcus agalactiae serine/threonine phosphatase. The subdomain conformation is coupled to the binding of a third metal ion

We solved the crystal structure of Streptococcus agalactiae serine/threonine phosphatase (SaSTP) using a combination of single-wavelength anomalous dispersion phasing and molecular replacement. The overall structure resembles that of previously characterized members of the PPM/PP2C STP family. The asymmetric unit contains four monomers and we observed two novel conformations for the flap domain among them. In one of these conformations, the enzyme binds three metal ions, whereas in the other it binds only two. The three-metal ion structure also has the active site arginine in a novel conformation. The switch between the two- and three-metal ion structures appears to be binding of another monomer to the active site of STP, which promotes binding of the third metal ion. This interaction may mimic the binding of a product complex, especially since the motif binding to the active site contains a serine residue aligning remarkably well with the phosphate found in the human STP structure.