The mechanism of Mycobacterium tuberculosis alkylhydroperoxidase AhpD as defined by mutagenesis,crystallography, and kinetics

AhpD, a protein with two cysteine residues, is required for physiological reduction of the Mycobacterium tuberculosis alkylhydroperoxidase AhpC. AhpD also has an alkylhydroperoxidase activity of its own. The AhpC/AhpD system provides critical antioxidant protection, particularly in the absence of the catalase-peroxidase KatG, which is suppressed in most isoniazid-resistant strains. Based on the crystal structure, we proposed recently a catalytic mechanism for AhpD involving a proton relay in which the Glu118 carboxylate group, via His137 and a water molecule, deprotonates the catalytic residue Cys133 (Nunn, C. M., Djordjevic, S., Hillas, P. J., Nishida, C., and Ortiz de Montellano, P. R. (2002) J. Biol. Chem. 277, 20033-20040). A possible role for His132 in subsequent formation of the Cys133-Cys130 disulfide bond was also noted. To test this proposed mechanism, we have expressed the H137F, H137Q, H132F, H132Q, E118F, E118Q, C133S, and C130S mutants of AhpD, determined the crystal structures of the H137F and H132Q mutants, estimated the pKa values of the cysteine residues, and defined the kinetic properties of the mutant proteins. The collective results strongly support the proposed catalytic mechanism for AhpD.     

Arfaptin 2 regulates the aggregation of mutant huntingtin protein

Huntington's disease (HD) is an inherited neurodegenerative disorder. Here we demonstrate that expression of arfaptin 2/POR1 (partner of Rac1) in cultured cells induces the formation of pericentriolar and nuclear aggregates, which morphologically resemble mutant huntingtin aggregates characteristic of HD. Endogenous arfaptin 2 localizes to aggregates induced by expression of an abnormal amino-terminal fragment of huntingtin that contains polyglutamine (polyQ) expansions. A dominant inhibitory mutant of arfaptin 2 inhibits aggregation of mutant huntingtin, but not in the presence of proteasome inhibitors. Using cell-free biochemical assays, we show that arfaptin 2 inhibits proteasome activity. Finally, we show that expression of arfaptin 2 is increased at sites of neurodegeneration and the protein localizes to huntingtin aggregates in HD transgenic mouse brains. Our data suggest that arfaptin 2 is involved in regulating huntingtin protein aggregation, possibly by impairing proteasome function.     

Analysis of similarity within 142 pairs of orthologous intergenic regions of Caenorhabditis elegans and Caenorhabditis briggsae

Patterns of similarity between genomes of related species reflect the distribution of selective constraint within DNA. We analyzed alignments of 142 orthologous intergenic regions of Caenorhabditis elegans and Caenorhabditis briggsae and found a mosaic pattern with regions of high similarity (phylogenetic footprints) interspersed with non-alignable sequences. Footprints cover ~20% of intergenic regions, often occur in clumps and are rare within 5′ UTRs but common within 3′ UTRs. The footprints have a higher ratio of transitions to transversions than expected at random and a higher GC content than the rest of the intergenic region. The number of footprints and the GC content of footprints within an intergenic region are higher when genes are oriented so that their 5′ ends form the boundaries of the intergenic region. Overall, the patterns and characteristics identified here, along with other comparative and experimental studies, suggest that many footprints have a regulatory function, although other types of function are also possible. These conclusions may be quite general across eukaryotes, and the characteristics of conserved regulatory elements determined from genomic comparisons can be useful in prediction of regulation sites within individual DNA sequences.     

Effect of Cardiac Arrest on Cognitive Impairment and Hippocampal Plasticity in Middle-Aged Rats

Cardiopulmonary arrest is a leading cause of death and disability in the United States that usually occurs in the aged population. Cardiac arrest (CA) induces global ischemia, disrupting global cerebral circulation that results in ischemic brain injury and leads to cognitive impairments in survivors. Ischemia-induced neuronal damage in the hippocampus following CA can result in the impairment of cognitive function including spatial memory. In the present study, we used a model of asphyxial CA (ACA) in nine month old male Fischer 344 rats to investigate cognitive and synaptic deficits following mild global cerebral ischemia. These experiments were performed with the goals of 1) establishing a model of CA in nine month old middle-aged rats; and 2) to test the hypothesis that learning and memory deficits develop following mild global cerebral ischemia in middle-aged rats. To test this hypothesis, spatial memory assays (Barnes circular platform maze and contextual fear conditioning) and field recordings (long-term potentiation and paired-pulse facilitation) were performed. We show that following ACA in nine month old middle-aged rats, there is significant impairment in spatial memory formation, paired-pulse facilitation n dysfunction, and a reduction in the number of non-compromised hippocampal Cornu Ammonis 1 and subiculum neurons. In conclusion, nine month old animals undergoing cardiac arrest have impaired survival, deficits in spatial memory formation, and synaptic dysfunction.     

Phosphate Homeostasis in Conditions of Phosphate Proficiency and Limitation in the Wild Type and the phoP Mutant of Streptomyces lividans

Phosphate, as a constituent of the high energy molecules, ATP/GTP and polyphosphate, plays a crucial role in most of the metabolic processes of living organisms. Therefore, the adaptation to low Pi availability is a major challenge for bacteria. In Streptomyces, this adaptation is tightly controlled by the two component PhoR/PhoP system. In this study, the free intracellular Pi, ATP, ADP and polyP content of the wild type and the phoP mutant strain of S. lividans TK24 were analyzed at discrete time points throughout growth in Pi replete and limited media. PolyP length and content was shown to be directly related to the Pi content of the growth medium. In Pi repletion, ATP and high molecular weight (HMW) polyP contents were higher in the phoP mutant than in the WT strain. This supports the recently proposed repressive effect of PhoP on oxidative phosphorylation. High oxidative phosphorylation activity might also have a direct or indirect positive impact on HMW polyP synthesis. In Pi sufficiency as in Pi limitation, the degradation of these polymers was shown to be clearly delayed in the phoP mutant, indicating PhoP dependent expression of the enzymes involved in this degradation. The efficient storage of Pi as polyphosphate and/or its inefficient degradation in Pi in the phoP mutant resulted in low levels of free Pi and ATP that are likely to be, at least in part, responsible for the very poor growth of this mutant in Pi limitation. Furthermore, short polyP was shown to be present outside the cell, tightly bound to the mycelium via electrostatic interactions involving divalent cations. Less short polyP was found to be associated with the mycelium of the phoP mutant than with that of the WT strain, indicating that generation and externalization of these short polyP molecules was directly or indirectly dependent on PhoP.     

Mass-spectrometry based minisequencing method for the rapid detection of drug resistance in Mycobacterium tuberculosis

A MALDI TOF MS based minisequencing method has been developed and applied for the analysis of rifampin (RIF)- and isoniazid (INH)-resistant M. tuberculosis strains. Eight genetic markers of RIF resistance-nucleotide polymorphisms located in RRDR of rpoB gene, and three of INH resistance including codon 315 of katG gene and − 8 and − 15 positions of the promoter region of fabG1-inhA operon were worked out. Based on the analysis of 100 M. tuberculosis strains collected from the Moscow region in 1997–2005 we deduced that 91% of RIF-resistant and 94% of INH-resistant strains can be identified using the technique suggested. The approach is rapid, reliable and allows to reveal the drug resistance of M. tuberculosis strains within 12 h after sample isolation.     

Widespread positive selection in synonymous sites of mammalian genes

Evolution of protein sequences is largely governed by purifying selection, with a small fraction of proteins evolving under positive selection. The evolution at synonymous positions in protein-coding genes is not nearly as well understood, with the extent and types of selection remaining, largely, unclear. A statistical test to identify purifying and positive selection at synonymous sites in protein-coding genes was developed. The method compares the rate of evolution at synonymous sites (Ks) to that in intron sequences of the same gene after sampling the aligned intron sequences to mimic the statistical properties of coding sequences. We detected purifying selection at synonymous sites in approximately 28% of the 1,562 analyzed orthologous genes from mouse and rat, and positive selection in approximately 12% of the genes. Thus, the fraction of genes with readily detectable positive selection at synonymous sites is much greater than the fraction of genes with comparable positive selection at nonsynonymous sites, i.e., at the level of the protein sequence. Unlike other genes, the genes with positive selection at synonymous sites showed no correlation between Ks and the rate of evolution in nonsynonymous sites (Ka), indicating that evolution of synonymous sites under positive selection is decoupled from protein evolution. The genes with purifying selection at synonymous sites showed significant anticorrelation between Ks and expression level and breadth, indicating that highly expressed genes evolve slowly. The genes with positive selection at synonymous sites showed the opposite trend, i.e., highly expressed genes had, on average, higher Ks. For the genes with positive selection at synonymous sites, a significantly lower mRNA stability is predicted compared to the genes with negative selection. Thus, mRNA destabilization could be an important factor driving positive selection in nonsynonymous sites, probably, through regulation of expression at the level of mRNA degradation and, possibly, also translation rate. So, unexpectedly, we found that positive selection at synonymous sites of mammalian genes is substantially more common than positive selection at the level of protein sequences. Positive selection at synonymous sites might act through mRNA destabilization affecting mRNA levels and translation.     

Allele-specific histone modifications regulate expression of the Dlk1-Gtl2 imprinted domain

Dlk1 and Gtl2 are reciprocally expressed imprinted genes located on mouse chromosome 12. The Dlk1-Gtl2 locus carries three differentially methylated regions (DMRs), which are methylated only on the paternal allele. Of these, the intergenic (IG) DMR, located 12 kb upstream of Gtl2, is required for proper imprinting of linked genes on the maternal chromosome, while the Gtl2 DMR, located across the promoter of the Gtl2 gene, is implicated in imprinting on both parental chromosomes. In addition to DNA methylation, modification of histone proteins is also an important regulator of imprinted gene expression. Chromatin immunoprecipitation was therefore used to examine the pattern of histone modifications across the IG and Gtl2 DMRs. The data show maternal-specific histone acetylation at the Gtl2 DMR, but not at the IG DMR. In contrast, only low levels of histone methylation were observed throughout the region, and there was no difference between the two parental alleles. An existing mouse line carrying a deletion/insertion upstream of Gtl2 is unable to imprint the Dlk1-Gtl2 locus properly and demonstrates loss of allele-specific methylation at the Gtl2 DMR. Further analysis of these animals now shows that the loss of allele-specific methylation is accompanied by increased paternal histone acetylation at the Gtl2 DMR, with the activated paternal allele adopting a maternal acetylation pattern. These data indicate that interactions between DNA methylation and histone acetylation are involved in regulating the imprinting of the Dlk1-Gtl2 locus.     

Prediction-based fingerprints of protein-protein interactions

The recognition of protein interaction sites is an important intermediate step toward identification of functionally relevant residues and understanding protein function, facilitating experimental efforts in that regard. Toward that goal, the authors propose a novel representation for the recognition of protein-protein interaction sites that integrates enhanced relative solvent accessibility (RSA) predictions with high resolution structural data. An observation that RSA predictions are biased toward the level of surface exposure consistent with protein complexes led the authors to investigate the difference between the predicted and actual (i.e., observed in an unbound structure) RSA of an amino acid residue as a fingerprint of interaction sites. The authors demonstrate that RSA prediction-based fingerprints of protein interactions significantly improve the discrimination between interacting and noninteracting sites, compared with evolutionary conservation, physicochemical characteristics, structure-derived and other features considered before. On the basis of these observations, the authors developed a new method for the prediction of protein-protein interaction sites, using machine learning approaches to combine the most informative features into the final predictor. For training and validation, the authors used several large sets of protein complexes and derived from them nonredundant representative chains, with interaction sites mapped from multiple complexes. Alternative machine learning techniques are used, including Support Vector Machines and Neural Networks, so as to evaluate the relative effects of the choice of a representation and a specific learning algorithm. The effects of induced fit and uncertainty of the negative (noninteracting) class assignment are also evaluated. Several representative methods from the literature are reimplemented to enable direct comparison of the results. Using rigorous validation protocols, the authors estimated that the new method yields the overall classification accuracy of about 74% and Matthews correlation coefficients of 0.42, as opposed to up to 70% classification accuracy and up to 0.3 Matthews correlation coefficient for methods that do not utilize RSA prediction-based fingerprints. The new method is available at http://sppider.cchmc.org.