No abstract

Cytoskeletal structures are dynamically remodeled with the aid of regulatory proteins. FtsZ (filamentation temperature-sensitive Z) is the bacterial homolog of tubulin that polymerizes into rings localized to cell-division sites, and the constriction of these rings drives cytokinesis. Here we investigate the mechanism by which the Bacillus subtilis cell-division inhibitor, MciZ (mother cell inhibitor of FtsZ), blocks assembly of FtsZ. The X-ray crystal structure reveals that MciZ binds to the C-terminal polymerization interface of FtsZ, the equivalent of the minus end of tubulin. Using in vivo and in vitro assays and microscopy, we show that MciZ, at substoichiometric levels to FtsZ, causes shortening of protofilaments and blocks the assembly of higher-order FtsZ structures. The findings demonstrate an unanticipated capping-based regulatory mechanism for FtsZ.T he discovery that bacteria have actin-, tubulin-, and intermediate filament-like proteins demonstrated that the cytoskeleton is an ancient invention, predating the divergence between prokaryotes and eukaryotes (1). The GTPase FtsZ (filamentation temperature-sensitive Z) was the first prokaryotic protein to be recognized as a cytoskeletal element (2, 3). FtsZ is a tubulin-like protein, which is widely conserved in bacteria and the main component of the bacterial cytokinesis machine, or "divisome." FtsZ self-assembles into single-stranded protofilaments and these associate further inside cells to form a superstructure known as the Z ring (4, 5). FtsZ alone can generate a constriction force to initiate division (6). The Z ring also provides a scaffold onto which several other components of the divisome-mostly cell wall synthesizing enzymes-are recruited and oriented so as to build the division septum, a cross-wall separating a progenitor cell into two isogenic daughter cells (7).FtsZ and tubulin share several essential properties: their assembly is cooperative, stimulated by GTP, and leads to GTP hydrolysis; they form dynamic polymers whose turnover is dependent on nucleotide hydrolysis (8); they use essentially the same bond for polymer formation (9); and recent evidence indicates that they undergo similar allosteric transitions upon polymerization (10, 11). Not surprisingly, however, the functional specialization of these proteins led to some significant differences between them, the most prominent being that FtsZ exists as single protofilaments, whereas tubulin always adopts a multifilament tubular structure. This difference in their higherorder structure implies that the reactions that lead to cooperativity and subunit turnover are likely different. It has also represented a significant technical challenge for the study of FtsZ. Because FtsZ filaments are smaller than the resolution of optical microscopy, so far it has been impossible to determine essential properties associated with its dynamic behavior.Similarly to actin filaments and microtubules, the assembly of FtsZ protofilaments into a Z ring is regulated by a number of proteins that bind directly...

No abstract

The enzymatic activity of thioredoxin reductase enzymes is endowed by at least two redox centers: a flavin and a dithiol/disulfide CXXC motif. The interaction between thioredoxin reductase and thioredoxin is generally species-specific, but the molecular aspects related to this phenomenon remain elusive. Here, we investigated the yeast cytosolic thioredoxin system, which is composed of NADPH, thioredoxin reductase (ScTrxR1), and thioredoxin 1 (ScTrx1) or thioredoxin 2 (ScTrx2). We showed that ScTrxR1 was able to efficiently reduce yeast thioredoxins (mitochondrial and cytosolic) but failed to reduce the human and Escherichia coli thioredoxin counterparts. To gain insights into this specificity, the crystallographic structure of oxidized ScTrxR1 was solved at 2.4 A resolution. The protein topology of the redox centers indicated the necessity of a large structural rearrangement for FAD and thioredoxin reduction using NADPH. Therefore, we modeled a large structural rotation between the two ScTrxR1 domains (based on the previously described crystal structure, PDB code 1F6M ). Employing diverse approaches including enzymatic assays, site-directed mutagenesis, amino acid sequence alignment, and structure comparisons, insights were obtained about the features involved in the species-specificity phenomenon, such as complementary electronic parameters between the surfaces of ScTrxR1 and yeast thioredoxin enzymes and loops and residues (such as Ser(72) in ScTrx2). Finally, structural comparisons and amino acid alignments led us to propose a new classification that includes a larger number of enzymes with thioredoxin reductase activity, neglected in the low/high molecular weight classification.

The phytopathogenic bacterium Xylella fastidiosa is the etiological agent of various plant diseases. To survive under oxidative stress imposed by the host, microorganisms express antioxidant proteins, including cysteine-based peroxidases named peroxiredoxins. This work is a comprehensive analysis of the catalysis performed by PrxQ from X. fastidiosa (XfPrxQ) that belongs to a peroxiredoxin class still poorly characterized and previously considered as moderately reactive toward hydroperoxides. Contrary to these assumptions, our competitive kinetics studies have shown that the second-order rate constants of the peroxidase reactions of XfPrxQ with hydrogen peroxide and peroxynitrite are in the order of 10 7 and 10 6 M ؊1 s ؊1 , respectively, which are as fast as the most efficient peroxidases. The XfPrxQ disulfides were only slightly reducible by dithiothreitol; therefore, the identification of a thioredoxin system as the probable biological reductant of XfPrxQ was a relevant finding. We also showed by site-specific mutagenesis and mass spectrometry that an intramolecular disulfide bond between Cys-47 and Cys-83 is generated during the catalytic cycle. Furthermore, we elucidated the crystal structure of XfPrxQ C47S in which Ser-47 and Cys-83 lie ϳ12.3 Å apart. Therefore, significant conformational changes are required for disulfide bond formation. In fact, circular dichroism data indicated that there was a significant redox-dependent unfolding of ␣-helices, which is probably triggered by the peroxidatic cysteine oxidation. Finally, we proposed a model that takes data from this work as well data as from the literature into account.

Oxidation of protein cysteine residues into sulfenic acid (Cys-SOH) and the subsequent S-glutathionylation of these residues during enzyme catalysis and redox signaling have been increasingly accepted as commonly occurring events in redox regulation [1][2][3][4][5][6][7][8][9]. This reversible mechanism is believed to play a regulatory role in enzyme catalysis and binding of transcription factors to DNA targets, among other processes. The first step in protein-Cys-SH oxidation generates Cys-SOH, which is prone to S-glutathionylation by The yeast 20S proteasome is subject to sulfhydryl redox alterations, such as the oxidation of cysteine residues (Cys-SH) into cysteine sulfenic acid (Cys-SOH), followed by S-glutathionylation (Cys-S-SG). Proteasome S-glutathionylation promotes partial loss of chymotrypsin-like activity and post-acidic cleavage without alteration of the trypsin-like proteasomal activity. Here we show that the 20S proteasome purified from stationaryphase cells was natively S-glutathionylated. Moreover, recombinant glutaredoxin 2 removes glutathione from natively or in vitro S-glutathionylated 20S proteasome, allowing the recovery of chymotrypsin-like activity and post-acidic cleavage. Glutaredoxin 2 deglutathionylase activity was dependent on its entry into the core particle, as demonstrated by stimulating S-glutathionylated proteasome opening. Under these conditions, deglutathionylation of the 20S proteasome and glutaredoxin 2 degradation were increased when compared to non-stimulated samples. Glutaredoxin 2 fragmentation by the 20S proteasome was evaluated by SDS-PAGE and mass spectrometry, and S-glutathionylation was evaluated by either western blot analyses with anti-glutathione IgG or by spectrophotometry with the thiol reactant 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole. It was also observed in vivo that glutaredoxin 2 was ubiquitinated in cellular extracts of yeast cells grown in glucose-containing medium. Other cytoplasmic oxido-reductases, namely thioredoxins 1 and 2, were also active in 20S proteasome deglutathionylation by a similar mechanism. These results indicate for the first time that 20S proteasome cysteinyl redox modification is a regulated mechanism coupled to enzymatic deglutathionylase activity.

Background:The type III secretion system allows bacteria to inject effectors directly into the host cytoplasm through a translocation pore. Results: Translocator proteins were characterized in lipid-bound and chaperone-associated forms. Conclusion: Pseudomonas translocators use different regions to recognize their common chaperone and the eukaryotic membrane. Significance: Mapping of key regions in translocators provides a starting point for the potential development of novel antibacterials.

No abstract