Three-dimensional structure of heat shock protein 90 from <Emphasis Type="Italic">Plasmodium falciparum</Emphasis>: molecular modelling approach to rational drug design against malaria

We have recently implicated heat shock protein 90 from Plasmodium falciparum (PfHsp90) as a potential drug target against malaria. Using inhibitors specific to the nucleotide binding domain of Hsp90, we have shown potent growth inhibitory effects on development of malarial parasite in human erythrocytes. To gain better understanding of the vital role played by PfHsp90 in parasite growth,we have modeled its three dimensional structure using recently described full length structure of yeast Hsp90.S equence similarity found between PfHsp90 and yeast Hsp90 allowed us to model the core structure with high confidence. The superimposition of the predicted structure with that of the template yeast Hsp90 structure reveals an RMSD of 3.31 Angstrom. The N-terminal and middle domains showed the least RMSD (1.76 Angstrom) while the more divergent C-terminus showed a greater RMSD (2.84 Angstrom) with respect to the template. The structure shows overall conservation of domains involved in nucleotide binding, ATPase activity, co-chaperone binding as well as inter-subunit interactions. Important co-chaperones known to modulate Hsp90 function in other eukaryotes are conserved in malarial parasite as well. An acidic stretch of amino acids found in the linker region, which is uniquely extended in PfHsp90 could not be modeled in this structure suggesting a flexible conformation. Our results provide a basis to compare the overall structure and functional pathways dependent on PfHsp90 in malarial parasite. Further analysis of differences found between human and parasite Hsp90 may make it possible to design inhibitors targeted specifically against malaria.     

Human down syndrome cell adhesion molecules (DSCAMs) are functionally conserved with Drosophila Dscam[TM1] isoforms in controlling neurodevelopment

Drosophila Down syndrome cell adhesion molecule (Dscam) potentially produces more than 150,000 cell adhesion molecules that share two alternative transmembrane/juxtamembrane (TM) domains, which dictate the dendrite versus axon subcellular distribution and function of different Dscam isoforms. Vertebrate genomes contain two closely related genes, DSCAM and DSCAM-Like1 (DSCAML1), which do not have extensive alternative splicing. We investigated the functional conservation between invertebrate Dscams and vertebrate DSCAMs by cross-species rescue assays and found that human DSCAM and DSCAML1 partially, but substantially, rescued the larval lethality of Drosophila Dscam mutants. Interestingly, both human DSCAM and DSCAML1 were targeted to the dendrites in Drosophila neurons, had synergistic rescue effects with Drosophila Dscam[TM2], and preferentially rescued the dendrite defects of Drosophila Dscam mutant neurons. Therefore, human DSCAM and DSCAML1 are functionally conserved with Drosophila Dscam[TM1] isoforms.     

Comparative shotgun proteomic analysis of Clostridium acetobutylicum from butanol fermentation using glucose and xylose

Background

Cytokinin is a plant hormone that plays a crucial role in several processes of plant growth and development. In recent years, major breakthroughs have been achieved in the elucidation of the metabolism, the signal perception and transduction, as well as the biological functions of cytokinin. An important activity of cytokinin is the involvement in chloroplast development and function. Although this biological function has already been known for 50 years, the exact mechanisms remain elusive.

Results

To elucidate the effects of altered endogenous cytokinin content on the structure and function of the chloroplasts, chloroplast subfractions (stroma and thylakoids) from transgenic Pssu-ipt and 35S:CKX1 tobacco (Nicotiana tabacum) plants with, respectively, elevated and reduced endogenous cytokinin content were analysed using two different 2-DE approaches. Firstly, thykaloids were analysed by blue-native polyacrylamide gel electrophoresis followed by SDS-PAGE (BN/SDS-PAGE). Image analysis of the gel spot pattern thus obtained from thylakoids showed no substantial differences between wild-type and transgenic tobacco plants. Secondly, a quantitative DIGE analysis of CHAPS soluble proteins derived from chloroplast subfractions indicated significant gel spot abundance differences in the stroma fraction. Upon identification by MALDI-TOF/TOF mass spectrometry, these proteins could be assigned to the Calvin-Benson cycle and photoprotective mechanisms.

Conclusion

Taken together, presented proteomic data reveal that the constitutively altered cytokinin status of transgenic plants does not result in any qualitative changes in either stroma proteins or protein complexes of thylakoid membranes of fully developed chloroplasts, while few but significant quantitative differences are observed in stroma proteins.     

Genome majority vote improves gene predictions

Recent studies have noted extensive inconsistencies in gene start sites among orthologous genes in related microbial genomes. Here we provide the first documented evidence that imposing gene start consistency improves the accuracy of gene start-site prediction. We applied an algorithm using a genome majority vote (GMV) scheme to increase the consistency of gene starts among orthologs. We used a set of validated Escherichia coli genes as a standard to quantify accuracy. Results showed that the GMV algorithm can correct hundreds of gene prediction errors in sets of five or ten genomes while introducing few errors. Using a conservative calculation, we project that GMV would resolve many inconsistencies and errors in publicly available microbial gene maps. Our simple and logical solution provides a notable advance toward accurate gene maps.     

Characterization of gp120 and its single-chain derivatives, gp120-CD4D12 and gp120-M9: implications for targeting the CD4i epitope in human immunodeficiency virus vaccine design

Single-chain derivatives of JRFL gp120 linked to the first two domains of human CD4 (gp120-CD4D12) or to the CD4 miniprotein analog CD4M9 (gp120-M9), have been constructed. Biacore studies revealed that gp120-CD4D12 and gp120-M9 bound to antibody 17b with dissociation constants of 0.8 and 25 nM, respectively, at pH 7.0, while gp120 alone did not bind. The binding of gp120-CD4D12 to 17b is not affected by the addition of excess soluble CD4D12, while the binding of gp120-M9 is enhanced. This finding indicates that the M9 component of the single chain interacts relatively weakly with gp120 and can be displaced by soluble CD4D12. Immunogenicity studies of gp120, gp120-CD4D12, and gp120-M9 were carried out with guinea pigs. All three molecules were highly immunogenic. The resulting antisera were examined for neutralizing activities against various human immunodeficiency virus type 1 isolates. Broadly neutralizing activity was observed only with sera generated against gp120-CD4D12. These antisera were depleted of anti-CD4D12 antibodies by being passed over a column containing immobilized CD4D12. The depleted sera showed a loss of broadly neutralizing activity. Sera that were affinity purified over a column containing immobilized gp120-M9 also lacked such neutralizing activity. This finding suggests that the broadly neutralizing response observed is exclusively due to anti-CD4 antibodies. Competition experiments showed that only antisera generated against gp120-CD4D12 competed with the CD4i antibody 17b and that this activity was not affected by depletion of anti-CD4 antibodies. The data indicate that although antibodies targeting the CD4i epitope were generated by the gp120-CD4D12 immunogen, these antibodies were nonneutralizing.     

Polypeptide substrate recognition by calnexin requires specific conformations of the calnexin protein

Calnexin is an endoplasmic reticulum chaperone that binds to substrates containing monoglucosylated oligosaccharides. Whether calnexin can also directly recognize polypeptide components of substrates is controversial. We found that calnexin displayed significant conformational lability for a chaperone and that heat treatment and calcium depletion induced the formation of calnexin dimers and higher order oligomers. These conditions enhanced the chaperone activity of calnexin toward glycosylated and non-glycosylated major histocompatibility complex (MHC) class I heavy chains, and enhanced calnexin binding to MHC class I heavy chains. In contrast to these observations, calnexin binding to oligosaccharide substrates has been reported to be impaired under calcium-depleting conditions. Calnexin dimers were induced in HeLa cells upon heat shock and under calcium-depleting conditions, and heat shock enhanced calnexin binding to MHC class I heavy chains in HeLa cells. Virus-induced endoplasmic reticulum stress also resulted in the appearance of calnexin dimers. Tunicamycin treatment of HeLa cells induced a slow accumulation of calnexin dimers, the appearance of which correlated with enhanced calnexin binding to deglycosylated MHC class I heavy chains. In vitro, the presence of calnexin-specific oligosaccharides inhibited the formation of calnexin dimers and higher order structures. Together, these data indicate that polypeptide binding is favored by conditions that induce partial unfolding of calnexin monomers, whereas oligosaccharide binding is favored by conditions that enhance the structural stability (folding) of calnexin monomers. Conditions that induce the calnexin "polypeptide-binding" conformation also induce self-association of calnexin if the concentration is sufficiently high; however, calnexin dimerization/oligomerization per se is not essential for polypeptide substrate binding.     

Attempts to delineate the relative contributions of changes in hydrophobicity and packing to changes in stability of ribonuclease S mutants

While the hydrophobic driving force is thought to be a major contributor to protein stability, it is difficult to experimentally dissect out its contribution to the overall free energy of folding. We have made large to small substitutions of buried hydrophobic residues at positions 8 and 13 in the peptide/protein complex, RNase-S, and have characterized the structures by X-ray crystallography. The thermodynamics of association of these mutant S peptides with S protein was measured in the presence of different concentrations of methanol and ethanol. The reduction in the strength of the hydrophobic driving force in the presence of these organic solvents was estimated from surface-tension data as well as from the dependence of the DeltaC(p) of protein/peptide binding on the alcohol concentration. The data indicated a decrease in the strength of the hydrophobic driving force of about 30-40% over a 0-30% range of the alcohol concentration. We observe that large to small substitutions destabilize the protein. However, the amount of destabilization, relative to the wild type, is independent of the alcohol concentration over the range of alcohol concentrations studied. The data clearly indicate that decreased stability of the mutants is primarily due to the loss of packing interactions rather than a reduced hydrophobic driving force and suggest a value of the hydrophobic driving force of less than 18 cal mol(-)(1) A(2).     

Protein stabilization by introduction of cross-strand disulfides

Disulfides cross-link residues in a protein that are separated in primary sequence and stabilize the protein through entropic destabilization of the unfolded state. While the removal of naturally occurring disulfides leads to protein destabilization, introduction of engineered disulfides does not always lead to significant stabilization of a protein. We have analyzed naturally occurring disulfides that span adjacent antiparallel strands of beta sheets (cross-strand disulfides). Cross-strand disulfides have recently been implicated as redox-based conformational switches in proteins such as gp120 and CD4. The propensity of these disulfides to act as conformational switches was postulated on the basis of the hypothesis that this class of disulfide is conformationally strained. In the present analysis, there was no evidence to suggest that cross-strand disulfides are more strained compared to other disulfides as assessed by their torsional energy. It was also observed that these disulfides occur solely at non-hydrogen-bonded (NHB) registered pairs of adjacent antiparallel strands and not at hydrogen-bonded (HB) positions as suggested previously. One of the half-cystines involved in cross-strand disulfide formation often occurs at an edge strand. Experimental confirmation of the stabilizing effects of such disulfides was carried out in Escherichia coli thioredoxin. Four pairs of cross-strand cysteines were introduced, two at HB and two at NHB pairs. Disulfides were formed in all four cases. However, as predicted from our analysis, disulfides at NHB positions resulted in an increase in melting temperature of 7-10 degrees C, while at HB positions there was a corresponding decrease of -7 degrees C. The reduced state of all proteins had similar stability.