Band 390, Heft 11

No abstract available

Bacterial chemotaxis is mediated by two reversible protein modification chemistries: phosphorylation and carboxyl methylation. Attractants bind to membrane chemoreceptors that control the activity of a protein kinase which acts in turn to control flagellar motor activity. Coordinate changes in receptor carboxyl methylation provide a negative feedback mechanism that serves a memory function. Protein carboxyl methylation might play an analogous role in the nervous system. Two protein carboxyl methyltransferases serve to regulate signal transduction pathways in eukaryotic cells. One is highly expressed in the Purkinje layer of the cerebellum where it methyl esterifies prenylated cysteine residues at the carboxyl-termini of Ras-related and heterotrimeric G-proteins. The other is abundant throughout the brain where it methylates the carboxyl-terminus of protein phosphatase 2A. The phosphatase methyltransferase and the protein methylesterase that reverses phosphatase methylation are structurally related to the corresponding bacterial chemotaxis methylating and demethylating enzymes. Recent results indicate that deficiencies in phosphatase methylation play an important role in the etiology of Alzheimer's disease.

Bacterial chemotaxis represents one of the simplest and best studied examples of unicellular behavior. Chemotaxis allows swimming bacterial cells to follow chemical gradients in the environment by performing temporal comparisons of ligand concentrations. The process of chemotaxis in the model bacterium Escherichia coli has been studied in great molecular detail over the past 40 years, using a large range of experimental tools to investigate physiology, genetics and biochemistry of the system. The abundance of quantitative experimental data enabled detailed computational modeling of the pathway and theoretical analyses of such properties as robustness and signal amplification. Because of the temporal mode of gradient sensing in bacterial chemotaxis, molecular memory is an essential component of the chemotaxis pathway. Recent studies suggest that the memory time scale has been evolutionary optimized to perform optimal comparisons of stimuli while swimming in the gradient. Moreover, noise in the adaptation system, which results from variations of the adaptation rate both over time and among cells, might be beneficial for the overall chemotactic performance of the population.

The flagellate Euglena gracilis contains as photoreceptor complex a heterotetrameric light-sensitive adenylyl cyclase (AC), consisting of the flavoproteins PACα and PACβ. Previously, we demonstrated the functional expression of PACα and PACβ in oocytes from Xenopus laevis and of PACα in different animal cell types. Both yielded a blue light-induced increase of cellular [cAMP]. Here, we report that the action spectrum of PACα is flavoprotein-typical, with maxima at ∼380 and ∼470 nm. Mutational analysis of PACα yields a model for its structure and function. PACα shows a basal AC activity in the dark which is unaffected by mutating the conserved tyrosines in the two flavin-binding domains (F1, F2), Y60 in F1 and Y472 in F2. Y60 in F1 is, however, essential for photoactivation as light-stimulation of cyclase activity is completely lost in the F1 mutant Y60F. This effect does not occur in the respective mutation in F2 (Y472F). Mutating the two cyclase domains (C1, C2) indicated that C1 and C2 form a heterodimeric catalytic center as in mammalian class III cyclases. Interaction of C1 with C2 in the same molecule could be excluded as coexpression of non-functional C1 and C2 mutants restored light-induced cyclase activity. Our results strongly suggest an intermolecular dimerization of C1 and C2 domains on PACα for a functional enzyme.

Chromatin states profoundly determine and maintain gene activity and gene repression in eukaryotic organisms. Regulation of chromatin involves chromatin remodeling, chromatin modifications and exchange of chromatin components and is linked to DNA methylation in some cases. In plants and other organisms, chromatin proteins control many developmental pathways, integrate changes in the environment and can confer a cellular memory of these cues. Here, we review the molecular mechanisms that provide a dynamic regulation of chromatin in a cell. In addition, we discuss how chromatin needs to be flexibly regulated during plant growth to confer stable expression states that can occasionally be reset, e.g., owing to changes in the environment and progression of development.

Structural memory of aqueous systems, such as neat water and biomolecules, in an aqueous environment is strongly influenced by hydrogen bond dynamics. Vibrational spectroscopy in the femtosecond (fs) time domain is applied to map structural dynamics in real-time and identify underlying molecular interactions. Neat liquid water displays an ultrafast loss of structural memory with the fastest decay of structural correlations occurring in the sub-100 fs regime. Both OH stretching and bending excitations of water molecules decay on a subpicosecond time scale, followed by dissipation of excess energy in the hydrogen bond network within a few picoseconds. Water shells around fully hydrated DNA show similar although slightly slower dynamics. A detailed study of hydration shells around ionic phosphate groups in the DNA backbone demonstrates a strong phosphate-water coupling and a subpicosecond rearrangement of hydrogen bonds upon energy disposal. Hydration shells serve as primary heat sinks for excess energy.

Optical data recording is one of the most widely used and efficient systems of memory in the non-living world. The application of color centers in this context offers not only systems of high speed in writing and read-out due to a high degree of parallelism in data handling but also a possibility to set up models of neural networks. In this way, systems with a high potential for image processing, pattern recognition and logical operations can be constructed. A limitation to storage density is given by the diffraction limit of optical data recording. It is shown that this limitation can at least in principle be overcome by the principle of spectral hole burning, which results in systems of storage capacities close to the human brain system.

Transcription factor regulation of gene expression and chromatin-controlled epigenetic memory systems are closely cooperating in establishing the pluripotent state of embryonic stem (ES) cells and maintaining cell fate decisions throughout development of an organism. A thorough understanding of the regulatory transcriptional circuitry that rules the underlying plastic yet heritable gene expression programs in ES cells is of great importance. With the advent of next-generation sequencing technologies facilitating the quantitative assessment of functional genomics assays it is now feasible to interrogate transcription networks at a genome-wide scale. Here, we discuss the application of next-generation sequencing in elucidating the molecular mechanisms underlying ES cell function.

The orchestrated expression of genes is essential for the development and survival of every organism. In addition to the role of transcription factors, the availability of genes for transcription is controlled by a series of proteins that regulate epigenetic chromatin remodeling. The two most studied epigenetic phenomena are DNA methylation and histone-tail modifications. Although a large body of literature implicates the deregulation of histone acetylation and DNA methylation with the pathogenesis of cancer, recently epigenetic mechanisms have also gained much attention in the neuroscientific community. In fact, a new field of research is rapidly emerging and there is now accumulating evidence that the molecular machinery that regulates histone acetylation and DNA methylation is intimately involved in synaptic plasticity and is essential for learning and memory. Importantly, dysfunction of epigenetic gene expression in the brain might be involved in neurodegenerative and psychiatric diseases. In particular, it was found that inhibition of histone deacetylases attenuates synaptic and neuronal loss in animal models for various neurodegenerative diseases and improves cognitive function. In this article, we will summarize recent data in the novel field of neuroepigenetics and discuss the question why epigenetic strategies are suitable therapeutic approaches for the treatment of brain diseases.

Saccharomyces cerevisiae calcineurin (CN) consists of a catalytic subunit CNA1 or CNA2 and a regulatory subunit CNB1. The kinetics of activation of yeast CN holoenzymes and their catalytic domains by Mn 2+ were investigated. We report that the in vitro phosphatase reaction activated by Mn 2+ typically has a pronounced initial lag phase caused by slow conformational rearrangement of the holoenzyme-Mn 2+ . A similar lag phase was detected using just the catalytic domain of yeast CN, indicating that the slowness of Mn 2+ -induced conformational change of CN results from a rearrangement within the catalytic domain. The Mn 2+ -activation of CN was reversible. The dissociation constant of the CN heterodimer containing the CNA2 subunit in the presence of Mn 2+ was 3-fold higher than that of CN containing the CNA1 subunit and that of the catalytic domains of CNA1 and CNA2, pointing to differences between the residues surrounding the Mn 2+ -binding sites of CNA1 and CNA2.

The periplasmic ligand-binding protein ChoX is part of the ABC transport system ChoVWX that imports choline as a nutrient into the soil bacterium Sinorhizobium meliloti . We have recently reported the crystal structures of ChoX in complex with its ligands choline and acetylcholine and the structure of a fully closed but substrate-free state of ChoX. This latter structure revealed an architecture of the ligand-binding site that is superimposable to the closed, ligand-bound form of ChoX. We report here the crystal structure of ChoX in an unusual, ligand-free conformation that represents a semi-closed form of ChoX. The analysis revealed a subdomain movement in the N-lobe of ChoX. Comparison with the two well-characterized substrate binding proteins, MBP and HisJ, suggests the presence of a similar subdomain in these proteins.

Glyoxalase II (GloII) is a ubiquitous thioester hydrolase catalyzing the last step of the glutathione-dependent conversion of 2-oxoaldehydes to 2-hydroxycarboxylic acids. Here, we present a detailed structure-function analysis of cGloII from the malaria parasite Plasmodium falciparum . The activity of the enzyme was salt-sensitive and pH-log k cat and pH-log k cat / K m profiles revealed acid-base catalysis. An acidic p K a app value of approximately 6 probably reflects hydroxide formation at the metal center. The glutathione-binding site was analyzed by site-directed mutagenesis. Substitution of residue Arg 154 caused a 2.5-fold increase of K m app , whereas replacements of Arg 257 or Lys 260 were far more detrimental. Although the glutathione-binding site and the catalytic center are separated, six of six single mutations at the substrate-binding site decreased the k cat app value. Furthermore, product inhibition studies support a Theorell-Chance Bi Bi mechanism with glutathione as the second product. We conclude that the substrate is predominantly bound via ionic interactions with the conserved residues Arg 257 and Lys 260 , and that correct substrate binding is a pH- and salt-dependent rate-limiting step for catalysis. The presented mechanistic model is presumably also valid for GloII from many other organisms. Our study could be valuable for drug development strategies and enhances the understanding of the chemistry of binuclear metallohydrolases.

In genome-wide screens we studied CA/C1 peptidases of malaria-causing plasmodia and their hosts (man and mouse). For Plasmodium falciparum and P. berghei , several new CA/C1 peptidase genes encoding proteases of the L- and B-family with specific promoter modules were identified. In addition, two new human CA/C1 peptidase loci and one new mouse gene locus were found; otherwise, the sets of CA/C1 peptidase genes in man and mouse seem to be complete now. In each species studied there is a multitude of CA/C1 peptidases with lysosomal localization signals and partial functional overlap according to similar but subfamily-specific structures. Individual target structures in plasmodia include residues specifically different in CA/C1 peptidase subsite 2. This is of medical interest considering CA/C1 peptidase inhibition for chemotherapy in malaria, malignancies and other diseases. Promoter structures and mRNA regulation differ widely among CA/C1 peptidase subfamilies and between mammals and plasmodia. We characterized promoter modules conserved in mouse and man for the CA/C1 peptidase families B and L (with the L-like subfamily, F-like subfamily and mouse-specific J-like subfamily). RNA motif searches revealed conserved regulatory elements such as GAIT elements; plasmodial CA/C1 peptidase mRNA elements include ARE elements and mammalian mRNAs contain 15-lox DICE elements.

In preeclampsia and hemolysis, elevated liver enzymes and low platelet (HELLP) syndrome, impaired trophoblast invasion and excessive fibrin deposition in the placental intervillous space is associated with fetal compromise. However, little information is available whether modulation of placental protease expression – potentially causing impaired trophoblast invasion – is associated with the HELLP syndrome. Total RNA and protein were extracted from placental tissue from 11 females with HELLP syndrome and 8 controls matched for gestational age. mRNA expression of matrix metalloprotease (MMP) -2 and -9, tissue inhibitors of metalloprotease (TIMP) -1, -2, and -3, and urokinase-type plasminogen activator receptor (uPAR) was determined by Northern blotting. Protein expression of MMP-2 and -9, and TIMP-1 and -2 was detected by Western blotting and that of uPA, uPAR, and plasminogen activator inhibitor (PAI) -1 by ELISA. In patients with HELLP syndrome, mRNA expression of MMP-2 and TIMP-2 was decreased, whereas TIMP-1 and -3 levels were unchanged. MMP-9 and uPAR mRNA was undetectable in both groups. Protein expression of all investigated proteolytic factors remained unchanged. Our findings at the mRNA level suggest a decrease in matrix remodeling in placentae from patients with HELLP syndrome compared with control pregnancies, although this is not supported at the protein level.

The inhibition of human cysteine cathepsins B, L, S and K was evaluated by a set of hypervalent tellurium compounds (telluranes) comprising both organic and inorganic derivatives. All telluranes studied showed a time- and concentration-dependent irreversible inhibition of the cathepsins, and their second-order inactivation rate constants were determined. The organic derivatives were potent inhibitors of the cathepsins and clear specificities were detected, which were parallel to their known substrate specificities. In all cases, the activity of the tellurane-inhibited cathepsins was recovered by treatment of the inactivated enzymes with reducing agents. The maximum stoichiometry of the reaction between cysteine residues and telluranes were also determined. The presented data indicate that it is possible to design organic compounds with a tellurium(IV) moiety as a novel warhead that covalently modifies the catalytic cysteine, and which also form strong interactions with subsites of cathepsins B, L, S and K, resulting in more specific inhibition.