Vitor Hugo Balasco Serrão

Promiscuous interactions of human septins: The GTP binding domain of SEPT7 forms filaments within the crystal

SEPT7 G and SEPT7 G bind by molecular sieving (View interaction).

Formation of a Ternary Complex for Selenocysteine Biosynthesis in Bacteria

Background: Selenoprotein biosynthesis requires the interaction of tRNASec and specific enzymes that drive the synthesis of selenocysteine. Results: Formation of a molecular complex of selenophosphate synthetase, selenocysteine synthase, and tRNASec was identified and characterized. Conclusion: The ternary complex formation is necessary for selenoprotein synthesis. Significance: Our findings demonstrate the formation of a ternary complex and provide a possible scenario for selenium metabolism in bacteria. The synthesis of selenocysteine-containing proteins (selenoproteins) involves the interaction of selenocysteine synthase (SelA), tRNA (tRNASec), selenophosphate synthetase (SelD, SPS), a specific elongation factor (SelB), and a specific mRNA sequence known as selenocysteine insertion sequence (SECIS). Because selenium compounds are highly toxic in the cellular environment, the association of selenium with proteins throughout its metabolism is essential for cell survival. In this study, we demonstrate the interaction of SPS with the SelA-tRNASec complex, resulting in a 1.3-MDa ternary complex of 27.0 ± 0.5 nm in diameter and 4.02 ± 0.05 nm in height. To assemble the ternary complex, SPS undergoes a conformational change. We demonstrated that the glycine-rich N-terminal region of SPS is crucial for the SelA-tRNASec-SPS interaction and selenoprotein biosynthesis, as revealed by functional complementation experiments. Taken together, our results provide new insights into selenoprotein biosynthesis, demonstrating for the first time the formation of the functional ternary SelA-tRNASec-SPS complex. We propose that this complex is necessary for proper selenocysteine synthesis and may be involved in avoiding the cellular toxicity of selenium compounds.

Assembly stoichiometry of bacterial selenocysteine synthase and SelC (tRNAsec)

In bacteria selenocysteyl–tRNAsec (SelC) is synthesized by selenocysteine synthase (SelA). Here we show by fluorescence anisotropy binding assays and electron microscopical symmetry analysis that the SelA–tRNAsec binding stoichiometry is of one tRNAsec molecule per SelA monomer (1:1) rather than the 1:2 value proposed previously. Negative stain transmission electron microscopy revealed a D5 pointgroup symmetry for the SelA–tRNAsec assembly both with and without tRNAsec bound. Furthermore, SelA can associate forming a supramolecular complex of stacked decamer rings, which does not occur in the presence of tRNAsec. We discuss the structure–function relationships of these assemblies and their regulatory role in bacterial selenocysteyl–tRNAsec synthesis.

Spectroscopic and calorimetric assays reveal dependence on dCTP and two metals (Zn2 + + Mg2 +) for enzymatic activity of Schistosoma mansoni deoxycytidylate (dCMP) deaminase

The parasite Schistosoma mansoni possess all pathways for pyrimidine biosynthesis, whereby deaminases play an essential role in the thymidylate cycle, a crucial step to controlling the ratio between cytidine and uridine nucleotides. In this study, we heterologously expressed and purified the deoxycytidylate (dCMP) deaminase from S. mansoni to obtain structural, biochemical and kinetic information. Small-angle X-ray scattering of this enzyme showed that it is organized as a hexamer in solution. Isothermal titration calorimetry was used to determine the kinetic constants for dCMP-dUMP conversion and the role of dCTP and dTTP in enzymatic regulation. We evaluated the metals involved in activating the enzyme and show for the first time the dependence of correct folding on the interaction of two metals. This study provides information that may be useful for understanding the regulatory mechanisms involved in the metabolic pathways of S. mansoni. Thus, improving our understanding of the function of these essential pathways for parasite metabolism and showing for the first time the hitherto unknown deaminase function in this parasite.

Structure and kinetics assays of recombinant Schistosoma mansoni dihydrofolate reductase

The parasite Schistosoma mansoni possesses all pathways for pyrimidine biosynthesis, in which dihydrofolate reductase (DHFR), thymidylate cycle participants, is essential for nucleotide metabolism to obtain energy and structural nucleic acids. Thus, DHFRs have been widely suggested as therapeutic targets for the treatment of infectious diseases. In this study, we expressed recombinant SmDHFR in a heterologous manner to obtain structural, biochemical and kinetic information. X-ray diffraction of recombinant SmDHFR at 1.95Å resolution showed that the structure exhibited the canonical DHFR fold. Isothermal titration calorimetry was used to determine the kinetic constants for NADP+ and dihydrofolate. Moreover, inhibition assays were performed using the commercial folate analogs methotrexate and aminopterin; these analogs are recognized as folate competitors and are used as chemotherapeutic agents in cancer and autoimmune diseases. This study provides information that may prove useful for the future discovery of novel drugs and for understanding these metabolic steps from this pathway of S. mansoni, thus aiding in our understanding of the function of these essential pathways for parasite metabolism.

The molecular structure of Schistosoma mansoni PNP isoform 2 provides insights into the nucleoside selectivity of PNPs.

Purine nucleoside phosphorylases (PNPs) play an important role in the blood fluke parasite Schistosoma mansoni as a key enzyme of the purine salvage pathway. Here we present the structural and kinetic characterization of a new PNP isoform from S. mansoni, SmPNP2. Thermofluorescence screening of different ligands suggested cytidine and cytosine are potential ligands. The binding of cytosine and cytidine were confirmed by isothermal titration calorimetry, with a KD of 27 μM for cytosine, and a KM of 76.3 μM for cytidine. SmPNP2 also displays catalytic activity against inosine and adenosine, making it the first described PNP with robust catalytic activity towards both pyrimidines and purines. Crystal structures of SmPNP2 with different ligands were obtained and comparison of these structures with the previously described S. mansoni PNP (SmPNP1) provided clues for the unique capacity of SmPNP2 to bind pyrimidines. When compared with the structure of SmPNP1, substitutions in the vicinity of SmPNP2 active site alter the architecture of the nucleoside base binding site thus permitting an alternative binding mode for nucleosides, with a 180° rotation from the canonical binding mode. The remarkable plasticity of this binding site enhances our understanding of the correlation between structure and nucleotide selectivity, thus suggesting new ways to analyse PNP activity.

The unique tRNA Sec and its role in selenocysteine biosynthesis

Selenium (Se) is an essential trace element for several organisms and is mostly present in proteins as l-selenocysteine (Sec or U). Sec is synthesized on its l-seryl–tRNASec to produce Sec–tRNASec molecules by a dedicated selenocysteine synthesis machinery and incorporated into selenoproteins at specified in-frame UGA codons. UGA–Sec insertion is signaled by an mRNA stem-loop structure called the SElenoCysteine Insertion Sequence (SECIS). tRNASec transcription regulation and folding have been described showing its importance to Sec biosynthesis. Here, we discuss structural aspects of Sec–tRNASec and its role in Sec biosynthesis as well as Sec incorporation into selenoproteins. Defects in the Sec biosynthesis or incorporation pathway have been correlated with pathological conditions.

Schistosoma mansoni purine and pyrimidine biosynthesis: structures and kinetic experiments in the search for the best therapeutic target

BACKGROUND Schistosoma mansoni is the etiological agent of schistosomiasis, a debilitating treatment neglected tropical disease that affects approximately 218 million people worldwide. Despite its importance, the treatment of schistosomiasis relies on a single drug, praziquantel. Some reports on the resistance of S. mansoni to this drug have stimulated efforts to develop new drugs to treat this disease. S. mansoni possesses all the same pyrimidine pathways (de novo, salvage and thymidylate cycles) as those of its host. The opposite scenario is true for purine metabolism, in which only the salvage pathway is present. These pathways have previously been proposed as potential drug targets. RESULTS Using modern molecular biology techniques, large-scale study of these pathways has become possible; 24 genes have been studied, and several protein structures and kinetic parameters have been determined. Unique characteristics of schistosomal enzymes have been obtained, which show that this organism possesses two isoforms of uridine phosphorylase (UP), which share 92% of identity. However, only one isoform has a canonical function, whereas the second isoform is expressed through all life stages and does not have a known function. In addition, the methylthioadenosine phosphorylase (MTAP) is one of the enzymes responsible for the previously described adenosine phosphorylase activity, thus representing one main difference between S. mansoni and its host. The study of adenine phosphoribosyltransferase has revealed possible differential expression of the APRT gene in females. This result is consistent with those obtained for the experimental treatment of schistosomiasis in monkeys with the adenosine analog tubercidin, which eliminates the disease mainly in females. CONCLUSION These important conclusions may aid in the development of new alternative drugs to treat schistosomiasis.

Structure analysis of capsid protein of Porcine circovirus type 2 from pigs with systemic disease

Economic losses with high mortality rate associated with Porcine circovirus type 2 (PCV2) is reported worldwide. PCV2 commercial vaccine was introduced in 2006 in U.S. and in 2008 in Brazil. Although PCV2 vaccines have been widely used, cases of PCV2 systemic disease have been reported in the last years. Eleven nursery or fattening pigs suffering from PCV2 systemic disease were selected from eight PCV2-vaccinated farms with historical records of PCV2 systemic disease in Southern Brazil. PCV2 genomes were amplified and sequenced from lymph node samples of selected pigs. The comparison among the ORF2 amino acid sequences of PCV2 isolates revealed three amino acid substitutions in the positions F57I, N178S and A190T, respectively. Using molecular modeling, a structural model for the capsid protein of PCV2 was built. Afterwards, the mutated residues positions were identified in the model. The structural analysis of the mutated residues showed that the external residue 190 is close to an important predicted region for antibodies recognition. Therefore, changes in the viral protein conformation might lead to an inefficient antibody binding and this could be a relevant mechanism underlying the recent vaccine failures observed in swine farms in Brazil.

Investigation of Escherichia coli Selenocysteine Synthase (SelA) Complex Formation Using Cryo-Electron Microscopy (Cryo-EM)

Incorporation of selenocysteine (Sec U) into proteins is directed by a in-frame UGA codon in all domains of life. In Bacteria, Sec biosynthesis and incorporation involves the interaction of Selenocysteine Synthase (SelA), tRNA (SelC or tRNA Sec ), Selenophosphate Synthetase (SPS), a specific elongation factor known as SelB and the specific mRNA structure SElenocysteine Insertion Sequence (SECIS), forming a complex molecular machinery. SelA is a homodecamer complex responsible for Ser-Sec conversion from selenophosphate delivered by SPS and seryl-tRNA sec , which differs from seryl-tRNA ser by its long variable arm and the UGA-codon. The specific mRNA sequence known as SElenoCysteine Insertion Sequence forms a hairpin-like secondary structure and is recognized by SelB for Sec incorporation in the nascent peptide [1-3]. Since selenium compounds are highly toxic in cellular environment, selenium association with proteins complexes throughout its metabolism is suggested to be essential for cell survival. However, macromolecular interactions between the different proteins have not yet been characterized.