Cytological and biochemical characterization of the FtsA cell division protein of Bacillus subtilis

The actin-like protein FtsA is present in many eubacteria, and genetic experiments have shown that it plays an important, sometimes essential, role in cell division. Here, we show that Bacillus subtilis FtsA is targeted to division sites in both vegetative and sporulating cells. As in other organisms FtsA is probably recruited immediately after FtsZ. In sporulating cells of B. subtilis FtsZ is recruited to potential division sites at both poles of the cell, but asymmetric division occurs at only one pole. We have now found that FtsA is recruited to only one cell pole, suggesting that it may play an important role in the generation of asymmetry in this system. FtsA is present in much higher quantities in B. subtilis than in Escherichia coli, with approximately one molecule of FtsA for five of FtsZ. This means that there is sufficient FtsA to form a complete circumferential ring at the division site. Therefore, FtsA may have a direct structural role in cell division. We have purified FtsA and shown that it behaves as a dimer and that it has both ATP-binding and ATP-hydrolysis activities. This suggests that ATP hydrolysis by FtsA is required, together with GTP hydrolysis by FtsZ, for cell division in B. subtilis (and possibly in most eubacteria).

Fate of the SpoIIAB*-ADP liberated after SpoIIAB phosphorylates SpoIIAA of Bacillus subtilis

Phosphorylation of SpoIIAA catalyzed by SpoIIAB helps to regulate the first sporulation-specific sigma factor, sigma(F), of Bacillus subtilis. The steady-state rate of phosphorylation is known to be exceptionally slow and to be limited by the return of the protein kinase, SpoIIAB, to a catalytically active state. Previous work from this laboratory has suggested that, after catalyzing the phosphorylation, SpoIIAB is in a form (SpoIIAB*) that does not readily release ADP. We now show that the rate of release of ADP from the SpoIIAB*-ADP complex was much diminished by the presence of unreacted SpoIIAA, suggesting that SpoIIAA can form a long-lived ternary complex with SpoIIAB*-ADP in which the SpoIIAB* form is stabilized. In kinetic studies of the phosphorylation of SpoIIAA, the ternary complex SpoIIAA-SpoIIAB*-ADP could be distinguished from the short-lived complex SpoIIAA-SpoIIAB-ADP, which can be readily produced in the absence of an enzymatic reaction.

Direct interaction between the cell division protein FtsZ and the cell differentiation protein SpoIIE

SpoIIE is a bifunctional protein with two critical roles in the establishment of cell fate in Bacillus subtilis. First, SpoIIE is needed for the normal formation of the asymmetrically positioned septum that forms early in sporulation and separates the mother cell from the prespore compartment. Secondly, SpoIIE is essential for the activation of the first compartment-specific transcription factor sigma(F) in the prespore. After initiation of sporulation, SpoIIE localizes to the potential asymmetric cell division sites near one or both cell poles. Localization of SpoIIE was shown to be dependent on the essential cell division protein FtsZ. To understand how SpoIIE is targeted to the asymmetric septum we have now analysed its interaction with FtsZ in vitro. Using the yeast two-hybrid system and purified FtsZ, and full-length and truncated SpoIIE proteins, we demonstrate that the two proteins interact directly and that domain II and possibly domain I of SpoIIE are required for the interaction. Moreover, we show that SpoIIE interacts with itself and suggest that this self-interaction plays a role in assembly of SpoIIE into the division machinery.

Genotype, phenotype, and protein structure in a regulator of sporulation: effects of mutations in the spoIIAA gene of Bacillus subtilis

SpoIIAA, a phosphorylatable protein, is essential to the regulation of sigmaF, the first sporulation-specific transcription factor of Bacillus subtilis. The solution structure of SpoIIAA has recently been published. Here we examine four mutant SpoIIAA proteins and correlate their properties with the phenotypes of the corresponding B. subtilis mutant strains. Two of the mutations severely disrupted the structure of the protein, a third greatly diminished the rate of its phosphorylation and abolished dephosphorylation, and the fourth left phosphorylation unaffected but reduced the rate of dephosphorylation about 10-fold.

The Bacillus subtilis regulator protein SpoIIE shares functional and structural similarities with eukaryotic protein phosphatases 2C

Dephosphorylation of SpoIIAA-P by SpoIIE is strictly dependent on the presence of the bivalent metal ions Mn2+ or Mg2+. Replacement by Ala of one of the four Asp residues, invariant in all representatives of protein phosphatase 2C, completely abolished the SpoIIE phosphatase activity in vitro, whilst replacement of the Asp residues by another acidic amino acid, Glu, had varying effects on the activities of the resulting mutated proteins. D610E and D795E exhibited some residual activity while D628E and D745E were without enzymatic activity. The results suggest that the functional model in which metal-associated water molecules are involved in the dephosphorylation reaction catalyzed by human protein phosphatase 2C alpha can also be applied to the bacterial protein phosphatase 2C-like protein.

Purification, kinetic properties, and intracellular concentration of SpoIIE, an integral membrane protein that regulates sporulation in Bacillus subtilis

SpoIIE is a bifunctional protein which controls sigmaF activation and formation of the asymmetric septum in sporulating Bacillus subtilis. The spoIIE gene of B. subtilis has now been overexpressed in Escherichia coli, and SpoIIE has been purified by anion-exchange chromatography and affinity chromatography. Kinetic studies showed that the rate of dephosphorylation of SpoIIAA-P by purified SpoIIE in vitro was 100 times greater, on a molar basis, than the rate of phosphorylation of SpoIIAA by SpoIIAB. The intracellular concentrations of SpoIIE and SpoIIAB were measured by quantitative immunoblotting between 0 and 4 h after the beginning of sporulation. The facts that these concentrations were very similar at hour 2 and that SpoIIE could be readily detected before asymmetric septation suggest that SpoIIE activity may be strongly regulated.

Development of superparamagnetic nanoparticles for MRI: effect of particle size, charge and surface nature on biodistribution

Twelve superparamagnetic Magnetite-Dextran (MD) nanoparticles potentially useful as contrast agents for Magnetic Resonance Imaging (MRI), with different sizes, charges and surface natures, were produced and internally labelled with (59)Fe in order to investigate the effect of their physicochemical properties on their biodistribution in mice. In a first step, neutral MD particles of a size 33-90.6 nm were studied. Next, the influence of charge was investigated with negative and positive particles (MDL, MDD, MDDEAE). The former (-25, -30 mV) were small, around 30 nm in size whereas the latter (+20 mV) were larger (104 nm). The effect of surface nature was evaluated using MD particles coated with polyoxyethylene-polyoxypropylene copolymers (Synperonic: these MDP particles were neutral and larger in size (65.9-76.4 nm). Experiments showed that 20 min post-injection (2 mg Fe/kg), liver uptake was enhanced when the mean diameter increased: 22% for the smallest and 42% for the largest. It was up to 3 X lower for electrically neutral particles than for charged particles. Coated particles presented higher vascular persistence. The diagnostic potential for liver, lymph node or vascular imaging were discussed.

[Transmission electron microscopy in the structural study of superparamagnetic contrast agents for MRI]

Iron oxide nanoparticles with different crystal sizes and uniform dextran coating were synthesized and analyzed by T.E.M.. The iron oxide core dimension and homogeneity of the preparation were correlated to magnetic properties. The increasing Fe/Dextran ratio used for the synthesis was well correlated with the mean diameter and the magnetic susceptibility. The comparison of the crystal size with the particle size determined by nanosizer in solution suggest that particles consist in nanoaggregates of many crystal subunits.

Liver-directed superparamagnetic iron oxide: quantitation of T2 relaxation effects

A new liver-directed superparamagnetic iron oxide nanoparticle preparation, MDL, is described. MDL is derived from previously developed MD particles only by modification of the characteristics of the coating:chemical structure and charge. The biodistribution, pharmacokinetics, and intratissular localization of both 59Fe-labeled MD and MDL particles were analyzed. R2 relaxivities determined in aqueous solution are compared to measurements in liver tissue and to R2 of nanoparticles incorporated into synthetic microcapsules, which represent a simplified cell pattern. T2 relaxation effects of both preparations in liver tissue are discussed relative to physical parameters such as iron oxide core dimension, total particle size, and charge, and pharmacological properties such as biodistribution, pharmacokinetics, and extra/intracellular localization.