Plasmodium falciparum: sequence diversity and antibody recognition of the Merozoite surface protein-2 (MSP-2) in Brazilian Amazonia

The merozoite surface protein-2 (MSP-2) of Plasmodium falciparum comprises repeats flanked by dimorphic domains defining the allelic families FC27 and IC1. Here, we examined sequence diversity at the msp-2 locus in Brazil and its impact on MSP-2 antibody recognition by local patients. Only 25 unique partial sequences of msp-2 were found in 61 isolates examined. The finding of identical msp-2 sequences in unrelated parasites, collected 6-13 years apart, suggests that no major directional selection is exerted by variant-specific immunity in this malaria-endemic area. To examine antibody cross-reactivity, recombinant polypeptides derived from locally prevalent and foreign MSP-2 variants were used in ELISA. Foreign IC1-type variants, such as 3D7 (currently tested for human vaccination), were less frequently recognized than FC27-type and local IC1-type variants. Antibodies discriminated between local and foreign IC1-type variants, but cross-recognized structurally different local IC1-type variants. The use of evolutionary models of MSP-2 is suggested to design vaccines that minimize differences between local parasites and vaccine antigens.     

[S2], [S3], and [N3] coordination modes of hydrotris(thioxotriazolyl)borato. Zinc, nickel, cobalt, and bismuth complexes

The ligand hydrotris(1,4-dihydro-3-methyl-4-phenyl-5-thioxo-1,2,4-triazolyl)borato (Tr^Ph,Me) was synthetized as natrium salt and the complexes [Zn(Tr^Ph,Me)₂] · 7.5H₂O · 1.5CH₃CN (2a), [Zn(Tr^Ph,Me)₂] · 8DMF (2b), [Co(Tr^Ph,Me)₂] · 8DMF (3a), [Ni(Tr^Ph,Me)₂] · H₂O · 6DMSO (4a), [Bi(Tr^Ph,Me)₂]NO₃ (5), have been isolated and structurally characterized by X-ray diffraction. In the zinc derivatives the ligand adopts different denticity and coordination modes, η² and [S₂] for 2a and η³ and [N₃] for 2b, depending on the crystallization solvent, giving rise to tetrahedral and octahedral geometry, respectively. In the octahedral cobalt and nickel complexes the ligand is η³ and [N₃] coordinated whereas in the bismuth complex the η³ and [S₃] coordination is exhibited.     

Salts of cationic platinum dithiolenes with anionic platinum complexes: structural characterization of [Pt(Me2pipdt)2][Pt(SCN)4] (Me2pipdt = N,N-dimethyl-piperazine-2,3-dithione)

[Pt(Me₂pipdt)₂](BF₄)₂ salts [Me₂pipdt = N,N^′-dimethyl-piperazine-2,3-dithione] bearing cationic dithiolene complexes react with (Bu₄N)₂[Pt(X)₄] (X = SCN, CN) to form [Pt(Me₂pipdt)₂][Pt(SCN)₄ ] (1) and [Pt(Me₂pipdt)₂][Pt(CN)₄] (2) salts by metathesis. Black crystals of 1 have been structurally characterized showing that the two metals lie on inversion centers and exhibit a square planar coordination. The Pt-S bond distances in the anion complex (2.324(2) Å) are longer than in the cation complex (2.280(2) Å) whereas the C-S bond distances are shorter in SCN⁻ (average 1.669 Å) than in Me₂pipdt (average 1.694 Å). The chelating Me₂Pipdt ligand is found disordered in the λ/δ conformations with site occupancies of 50/50, respectively. The cation and anion complexes run parallel to a.     

The p21-activated protein kinase-related kinase Cla4 is a coincidence detector of signaling by Cdc42 and phosphatidylinositol 4-phosphate

Signal transduction pathways that co-regulate a given biological process often are organized into networks by molecules that act as coincidence detectors. Phosphoinositides and the Rho-type GTPase Cdc42 regulate overlapping processes in all eukaryotic cells. However, the coincidence detectors that link these pathways into networks remain unknown. Here we show that the p21-activated protein kinase-related kinase Cla4 of yeast integrates signaling by Cdc42 and phosphatidylinositol 4-phosphate (PI4P). We found that the Cla4 pleckstrin homology (PH) domain binds in vitro to several phosphoinositide species. To determine which phosphoinositides regulate Cla4 in vivo, we analyzed phosphatidylinositol kinase mutants (stt4, mss4, and pik1). This indicated that the plasma membrane pool of PI4P, but not phosphatidylinositol 4,5-bisphosphate or the Golgi pool of PI4P, is required for localization of Cla4 to sites of polarized growth. A combination of the Cdc42-binding and PH domains of Cla4 was necessary and sufficient for localization to sites of polarized growth. Point mutations affecting either domain impaired the ability of Cla4 to regulate cell morphogenesis and the mitotic exit network (localization of Lte1). Therefore, Cla4 must retain the ability to bind both Cdc42 and phosphoinositides, the hallmark of a coincidence detector. PI4P may recruit Cla4 to the plasma membrane where Cdc42 activates its kinase activity and refines its localization to cortical sites of polarized growth. In mammalian cells, the myotonic dystrophy-related Cdc42-binding kinase possesses p21-binding and PH domains, suggesting that this kinase may be a coincidence detector of signaling by Cdc42 and phosphoinositides.     

Evolutionary links as revealed by the structure of Thermotoga maritima S-adenosylmethionine decarboxylase

S-adenosylmethionine decarboxylase (AdoMetDC) is a critical regulatory enzyme of the polyamine biosynthetic pathway and belongs to a small class of pyruvoyl-dependent amino acid decarboxylases. Structural elucidation of the prokaryotic AdoMetDC is of substantial interest in order to determine the relationship between the eukaryotic and prokaryotic forms of the enzyme. Although both forms utilize pyruvoyl groups, there is no detectable sequence similarity except at the site of pyruvoyl group formation. The x-ray structure of the Thermatoga maritima AdoMetDC proenzyme reveals a dimeric protein fold that is remarkably similar to the eukaryotic AdoMetDC protomer, suggesting an evolutionary link between the two forms of the enzyme. Three key active site residues (Ser55, His68, and Cys83) involved in substrate binding, catalysis or proenzyme processing that were identified in the human and potato AdoMet-DCs are structurally conserved in the T. maritima AdoMetDC despite very limited primary sequence identity. The role of Ser55, His68, and Cys83 in the self-processing reaction was investigated through site-directed mutagenesis. A homology model for the Escherichia coli AdoMetDC was generated based on the structures of the T. maritima and human AdoMetDCs.     

Characterization of CA XIII, a novel member of the carbonic anhydrase isozyme family

The carbonic anhydrase (CA) gene family has been reported to consist of at least 11 enzymatically active members and a few inactive homologous proteins. Recent analyses of human and mouse databases provided evidence that human and mouse genomes contain genes for still another novel CA isozyme hereby named CA XIII. In the present study, we modeled the structure of human CA XIII. This model revealed a globular molecule with high structural similarity to cytosolic isozymes, CA I, II, and III. Recombinant mouse CA XIII showed catalytic activity similar to those of mitochondrial CA V and cytosolic CA I, with k(cat)/K(m) of 4.3 x 10(7) m(-1) s(-1), and k(cat) of 8.3 x 10(4) s(-1). It is very susceptible to inhibition by sulfonamide and anionic inhibitors, with inhibition constants of 17 nm for acetazolamide, a clinically used sulfonamide, and of 0.25 microm, for cyanate, respectively. Using panels of cDNAs we evaluated human and mouse CA13 gene expression in a number of different tissues. In human tissues, positive signals were identified in the thymus, small intestine, spleen, prostate, ovary, colon, and testis. In mouse, positive tissues included the spleen, lung, kidney, heart, brain, skeletal muscle, and testis. We also investigated the cellular and subcellular localization of CA XIII in human and mouse tissues using an antibody raised against a polypeptide of 14 amino acids common for both human and mouse orthologues. Immunohistochemical staining showed a unique and widespread distribution pattern for CA XIII compared with the other cytosolic CA isozymes. In conclusion, the predicted amino acid sequence, structural model, distribution, and activity data suggest that CA XIII represents a novel enzyme, which may play important physiological roles in several organs.     

Seeding specificity in amyloid growth induced by heterologous fibrils

Over residues 15-36, which comprise the H-bonded core of the amyloid fibrils it forms, the Alzheimer's disease plaque peptide amyloid beta (Abeta) possesses a very similar sequence to that of another short, amyloidogenic peptide, islet amyloid polypeptide (IAPP). Using elongation rates to quantify seeding efficiency, we inquired into the relationship between primary sequence similarity and seeding efficiency between Abeta-(1-40) and amyloid fibrils produced from IAPP as well as other proteins. In both a solution phase and a microtiter plate elongation assay, IAPP fibrils are poor seeds for Abeta-(1-40) elongation, exhibiting weight-normalized efficiencies of only 1-2% compared with Abeta-(1-40) fibrils. Amyloid fibrils of peptides with sequences completely unrelated to Abeta also exhibit poor to negligible seeding ability for Abeta elongation. Fibrils from a number of point mutants of Abeta-(1-40) exhibit intermediate seeding abilities for wild-type Abeta elongation, with differing efficiencies depending on whether or not the mutation is in the amyloid core region. The results suggest that amyloid fibrils from different proteins exhibit structural differences that control seeding efficiencies. Preliminary results also suggest that identical sequences can grow into different conformations of amyloid fibrils as detected by seeding efficiencies. The results have a number of implications for amyloid structure and biology.     

A thermal unfolding study of plastocyanin from the thermophilic cyanobacterium Phormidium laminosum

The thermal unfolding of the plastocyanin from Phormidium laminosum, a thermophilic cyanobacterium, is herein described. The main objective of this work is to identify structural factors responsible for the higher stability observed in proteins from thermophilic organisms. With the aid of fluorescence spectroscopy, EPR, and NMR, the factors influencing the unfolding process of the protein were investigated, and procedures for its study have been standardized. The different spectroscopic techniques used provided consistent results showing that the thermal unfolding of plastocyanin is irreversible under all the conditions investigated and that this irreversibility does not appear to be related to the presence of oxygen. The oxidized plastocyanin species has proven to be more stable than the reduced one, with respect to both the required temperature for protein unfolding (up to a 9 degrees C difference between the two forms) and the kinetics of the process. The behavior of this plastocyanin contrasts with that of other cupredoxins whose unfolding had previously been studied. The unfolding pH dependence and kinetic studies indicate a process with a tight control around the physiological pH in which plastocyanin plays its redox role and the protein's isoelectric point (5.2), suggesting a close compromise between function and stability.     

Active site structure of the aa3 quinol oxidase of Acidianus ambivalens

The membrane bound aa(3)-type quinol:oxygen oxidoreductase from the hyperthermophilic archaeon, Acidianus ambivalens, which thrives at a pH of 2.5 and a temperature of 80 degrees C, has several unique structural and functional features as compared to the other members of the heme-copper oxygen reductase superfamily, but shares the common redox-coupled, proton-pumping function. To better understand the properties of the heme a(3)-Cu(B) catalytic site, a resonance Raman spectroscopic study of the enzyme under a variety of conditions and in the presence of various ligands was carried out. Assignments of several heme vibrational modes as well as iron-ligand stretching modes are made to serve as a basis for comparing the structure of the enzyme to that of other oxygen reductases. The CO-bound oxidase has conformations that are similar to those of other oxygen reductases. However, the addition of CO to the resting enzyme does not generate a mixed valence species as in the bovine aa(3) enzyme. The cyanide complex of the oxidized enzyme of A. ambivalens does not display the high stability of its bovine counterpart, and a redox titration demonstrates that there is an extensive heme-heme interaction reflected in the midpoint potentials of the cyanide adduct. The A. ambivalens oxygen reductase is very stable under acidic conditions, but it undergoes an earlier alkaline transition than the bovine enzyme. The A. ambivalens enzyme exhibits a redox-linked reversible conformational transition in the heme a(3)-Cu(B) center. The pH dependence and H/D exchange demonstrate that the conformational transition is associated with proton movements involving a group or groups with a pK(a) of approximately 3.8. The observed reversibility and involvement of protons in the redox-coupled conformational transition support the proton translocation model presented earlier. The implications of such conformational changes are discussed in relation to general redox-coupled proton pumping mechanisms in the heme-copper oxygen reductases.