Perturbing the linker regions of the alpha-subunit of transducin: a new class of constitutively active GTP-binding proteins

The GDP-GTP exchange activity of the retinal G protein, transducin, is markedly accelerated by the photoreceptor rhodopsin in the first step of visual transduction. The x-ray structures for the alpha subunits of transducin (alpha(T)) and other G proteins suggest that the nucleotide-binding (Ras-like) domain and a large helical domain form a "clam shell" that buries the GDP molecule. Thus, receptor-promoted G protein activation may involve "opening the clam shell" to facilitate GDP dissociation. In this study, we have examined whether perturbing the linker regions connecting the Ras-like and helical domains of Galpha subunits gives rise to a more readily exchangeable state. The sole glycine residues in linkers 1 and 2 were individually changed to proline residues within an alpha(T)/alpha(i1) chimera (designated alpha(T)(*)). Both alpha(T)(*) linker mutants showed significant increases in their basal rates of GDP-GTP exchange when compared either to retinal alpha(T) or recombinant alpha(T)(*). The alpha(T)(*) linker mutants were responsive to aluminum fluoride, which binds to alpha-GDP complexes and induces changes in Switch 2. Although both linker mutants were further activated by light-activated rhodopsin together with the betagamma complex, their activation was not influenced by betagamma alone, arguing against the idea that the betagamma complex helps to pry apart the helical and Ras-like domains of Galpha subunits. Once activated, the alpha(T)(*) linker mutants were able to stimulate the cyclic GMP phosphodiesterase. Overall, these findings highlight a new class of activated Galpha mutants that constitutively exchange GDP for GTP and should prove valuable in studying different G protein-signaling systems.     

X-ray analysis of Mycobacterium smegmatis Dps and a comparative study involving other Dps and Dps-like molecules

The structure of the DNA binding protein from starved cells from Mycobacterium smegmatis has been determined in three crystal forms and has been compared with those of similar proteins from other sources. The dodecameric molecule can be described as a distorted icosahedron. The interfaces among subunits are such that the dodecameric molecule appears to have been made up of stable trimers. The situation is similar in the proteins from Escherichia coli and Agrobacterium tumefaciens, which are closer to the M.smegmatis protein in sequence and structure than those from other sources, which appear to form a dimer first. Trimerisation is aided in the three proteins by the additional N-terminal stretches that they possess. The M.smegmatis protein has an additional C-terminal stretch compared to other related proteins. The stretch, known to be involved in DNA binding, is situated on the surface of the molecule. A comparison of the available structures permits a delineation of the rigid and flexible regions in the molecule. The subunit interfaces around the molecular dyads, where the ferroxidation centres are located, are relatively rigid. Regions in the vicinity of the acidic holes centred around molecular 3-fold axes, are relatively flexible. So are the DNA binding regions. The crystal structures of the protein from M.smegmatis confirm that DNA molecules can occupy spaces within the crystal without disturbing the arrangement of the protein molecules. However, contrary to earlier suggestions, the spaces do not need to be between layers of protein molecules. The cubic form provides an arrangement in which grooves, which could hold DNA molecules, criss-cross the crystal.     

Bending stiffness of a crystalline actin bundle

The acrosomal process of the sperm of the horseshoe crab (Limulus polyphemus) is a unique crystalline actin bundle, consisting of multiple actin filaments cross-linked by the actin-bundling protein, scruin. For successful fertilization, the acrosomal bundle must penetrate through a 30 microm thick jelly coat surrounding the egg and thus it must be sufficiently stiff. Here, we present two measurements of the bending stiffness of a single crystalline bundle of actin. Results from these measurements indicate that the actin:scruin composite bundle has an average elastic modulus of 2 GPa, which is similar to that of a single actin filament, and a bending stiffness that is more than two orders of magnitude larger than that of a bundle of uncross-linked actin filaments due to stiffening by the scruin matrix.     

FAIR: A server for internal sequence repeats

An Internet computing server has been developed to identify all the occurrences of the internal sequence repeats in a protein and DNA sequences. Further, an option is provided for the users to check the occurrence(s) of the resultant sequence repeats in the other sequence and structure (Protein Data Bank) databases. The databases deployed in the proposed computing engine are up-to-date and thus the users will get the latest information available in the respective databases. The server is freely accessible over the World Wide Web (WWW). AVAILABILITY: http://bioserver1.physics.iisc.ernet.in/fair/     

Characterization of a mercury-reducing Bacillus cereus strain isolated from the Pulicat Lake sediments, south east coast of India

Pulicat Lake sediments are often severely polluted with the toxic heavy metal mercury. Several mercury-resistant strains of Bacillus species were isolated from the sediments and all the isolates exhibited broad spectrum resistance (resistance to both organic and inorganic mercuric compounds). Plasmid curing assay showed that all the isolated Bacillus strains carry chromosomally borne mercury resistance. Polymerase chain reaction and southern hybridization analyses using merA and merB3 gene primers/probes showed that five of the isolated Bacillus strains carry sequences similar to known merA and merB3 genes. Results of multiple sequence alignment revealed 99% similarity with merA and merB3 of TnMERI1 (class II transposons). Other mercury resistant Bacillus species lacking homology to these genes were not able to volatilize mercuric chloride, indicating the presence of other modes of resistance to mercuric compounds.     

Integration of a physiologically-based pharmacokinetic model with a whole-body,organ-resolved genome-scale model for characterization of ethanol and acetaldehyde metabolism

Ethanol is one of the most widely used recreational substances in the world and due to its ubiquitous use, ethanol abuse has been the cause of over 3.3 million deaths each year. In addition to its effects, ethanol’s primary metabolite, acetaldehyde, is a carcinogen that can cause symptoms of facial flushing, headaches, and nausea. How strongly ethanol or acetaldehyde affects an individual depends highly on the genetic polymorphisms of certain genes. In particular, the genetic polymorphisms of mitochondrial aldehyde dehydrogenase, ALDH2, play a large role in the metabolism of acetaldehyde. Thus, it is important to characterize how genetic variations can lead to different exposures and responses to ethanol and acetaldehyde. While the pharmacokinetics of ethanol metabolism through alcohol dehydrogenase have been thoroughly explored in previous studies, in this paper, we combined a base physiologically-based pharmacokinetic (PBPK) model with a whole-body genome-scale model (WBM) to gain further insight into the effect of other less explored processes and genetic variations on ethanol metabolism. This combined model was fit to clinical data and used to show the effect of alcohol concentrations, organ damage, ALDH2 enzyme polymorphisms, and ALDH2-inhibiting drug disulfiram on ethanol and acetaldehyde exposure. Through estimating the reaction rates of auxiliary processes with dynamic Flux Balance Analysis, The PBPK-WBM was able to navigate around a lack of kinetic constants traditionally associated with PK modelling and demonstrate the compensatory effects of the body in response to decreased liver enzyme expression. Additionally, the model demonstrated that acetaldehyde exposure increased with higher dosages of disulfiram and decreased ALDH2 efficiency, and that moderate consumption rates of ethanol could lead to unexpected accumulations in acetaldehyde. This modelling framework combines the comprehensive steady-state analyses from genome-scale models with the dynamics of traditional PK models to create a highly personalized form of PBPK modelling that can push the boundaries of precision medicine.     

Characterization of metabolism in the Fe(III)-reducing organism Geobacter sulfurreducens by constraint-based modeling

Geobacter sulfurreducens is a well-studied representative of the Geobacteraceae, which play a critical role in organic matter oxidation coupled to Fe(III) reduction, bioremediation of groundwater contaminated with organics or metals, and electricity production from waste organic matter. In order to investigate G. sulfurreducens central metabolism and electron transport, a metabolic model which integrated genome-based predictions with available genetic and physiological data was developed via the constraint-based modeling approach. Evaluation of the rates of proton production and consumption in the extracellular and cytoplasmic compartments revealed that energy conservation with extracellular electron acceptors, such as Fe(III), was limited relative to that associated with intracellular acceptors. This limitation was attributed to lack of cytoplasmic proton consumption during reduction of extracellular electron acceptors. Model-based analysis of the metabolic cost of producing an extracellular electron shuttle to promote electron transfer to insoluble Fe(III) oxides demonstrated why Geobacter species, which do not produce shuttles, have an energetic advantage over shuttle-producing Fe(III) reducers in subsurface environments. In silico analysis also revealed that the metabolic network of G. sulfurreducens could synthesize amino acids more efficiently than that of Escherichia coli due to the presence of a pyruvate-ferredoxin oxidoreductase, which catalyzes synthesis of pyruvate from acetate and carbon dioxide in a single step. In silico phenotypic analysis of deletion mutants demonstrated the capability of the model to explore the flexibility of G. sulfurreducens central metabolism and correctly predict mutant phenotypes. These results demonstrate that iterative modeling coupled with experimentation can accelerate the understanding of the physiology of poorly studied but environmentally relevant organisms and may help optimize their practical applications.     

Elevated Oxidative Stress and Decreased Antioxidant Function in the Human Hippocampus and Frontal Cortex with Increasing Age: Implications for Neurodegeneration in Alzheimer’s Disease

Oxidative stress and mitochondrial damage are implicated in the evolution of neurodegenerative diseases. Increased oxidative damage in specific brain regions during aging might render the brain susceptible to degeneration. Previously, we demonstrated increased oxidative damage and lowered antioxidant function in substantia nigra during aging making it vulnerable to degeneration associated with Parkinson's disease. To understand whether aging contributes to the vulnerability of brain regions in Alzheimer's disease, we assessed the oxidant and antioxidant markers, glutathione (GSH) metabolic enzymes, glial fibrillary acidic protein (GFAP) expression and mitochondrial complex I (CI) activity in hippocampus (HC) and frontal cortex (FC) compared with cerebellum (CB) in human brains with increasing age (0.01-80 years). We observed significant increase in protein oxidation (HC: p = 0.01; FC: p = 0.0002) and protein nitration (HC: p = 0.001; FC: p = 0.02) and increased GFAP expression (HC: p = 0.03; FC: p = 0.001) with a decreasing trend in CI activity in HC and FC compared to CB with increasing age. These changes were associated with a decrease in antioxidant enzyme activities, such as superoxide dismutase (HC: p = 0.005), catalase (HC: p = 0.02), thioredoxin reductase (FC: p = 0.04), GSH reductase (GR) (HC: p = 0.005), glutathione-s-transferase (HC: p = 0.0001; FC: p = 0.03) and GSH (HC: p = 0.01) with age. However, these parameters were relatively unaltered in CB. We suggest that the regions HC and FC are subjected to widespread oxidative stress, loss of antioxidant function and enhanced GFAP expression during aging which might make them more susceptible to deranged physiology and selective neuronal degeneration.