Structural basis for the binding specificity of human Recepteur d'Origine Nantais (RON) receptor tyrosine kinase to macrophage-stimulating protein.

Recepteur d'origine nantais (RON) receptor tyrosine kinase and its ligand, serum macrophage-stimulating protein (MSP), play important roles in inflammation, cell growth, migration, and epithelial to mesenchymal transition during tumor development. The binding of mature MSPαß (disulfide-linked α- and ß-chains) to RON ectodomain modulates receptor dimerization, followed by autophosphorylation of tyrosines in the cytoplasmic receptor kinase domains. Receptor recognition is mediated by binding of MSP ß-chain (MSPß) to the RON Sema. Here we report the structure of RON Sema-PSI-IPT1 (SPI1) domains in complex with MSPß at 3.0 Å resolution. The MSPß serine protease-like ß-barrel uses the degenerate serine protease active site to recognize blades 2, 3, and 4 of the ß-propeller fold of RON Sema. Despite the sequence homology between RON and MET receptor tyrosine kinase and between MSP and hepatocyte growth factor, it is well established that there is no cross-reactivity between the two receptor-ligand systems. Comparison of the structure of RON SPI1 in complex with MSPß and that of MET receptor tyrosine kinase Sema-PSI in complex with hepatocyte growth factor ß-chain reveals the receptor-ligand selectivity determinants. Analytical ultracentrifugation studies of the SPI1-MSPß interaction confirm the formation of a 1:1 complex. SPI1 and MSPαß also associate primarily as a 1:1 complex with a binding affinity similar to that of SPI1-MSPß. In addition, the SPI1-MSPαß ultracentrifuge studies reveal a low abundance 2:2 complex with ∼ 10-fold lower binding affinity compared with the 1:1 species. These results support the hypothesis that the α-chain of MSPαß mediates RON dimerization.

Structural reorganization of the interleukin-7 signaling complex.

We report here an unliganded receptor structure in the common gamma-chain (γ(c)) family of receptors and cytokines. The crystal structure of the unliganded form of the interleukin-7 alpha receptor (IL-7Rα) extracellular domain (ECD) at 2.15 Å resolution reveals a homodimer forming an "X" geometry looking down onto the cell surface with the C termini of the two chains separated by 110 Å and the dimer interface comprising residues critical for IL-7 binding. Further biophysical studies indicate a weak association of the IL-7Rα ECDs but a stronger association between the γ(c)/IL-7Rα ECDs, similar to previous studies of the full-length receptors on CD4(+) T cells. Based on these and previous results, we propose a molecular mechanism detailing the progression from the inactive IL-7Rα homodimer and IL-7Rα-γ(c) heterodimer to the active IL-7-IL-7Rα-γ(c) ternary complex whereby the two receptors undergo at least a 90° rotation away from the cell surface, moving the C termini of IL-7Rα and γ(c) from a distance of 110 Å to less than 30 Å at the cell surface. This molecular mechanism can be used to explain recently discovered IL-7- and γ(c)-independent gain-of-function mutations in IL-7Rα from B- and T-cell acute lymphoblastic leukemia patients. The mechanism may also be applicable to other γ(c) receptors that form inactive homodimers and heterodimers independent of their cytokines.

Acclimation and Compensating Metabolite Responses to UV-B Radiation in Natural and Transgenic Populus spp. Defective in Lignin Biosynthesis.

Plants have evolved to protect leaf mesophyll tissue from damage caused by UV-B radiation by producing an array of UV-absorbing secondary metabolites. Flavonoids (phenolic glycosides) and sinapate esters (hydroxycinnamates) have been implicated as UV-B protective compounds because of the accumulation in the leaf epidermis and the strong absorption in the wavelengths corresponding to UV. Environmental adaptations by plants also generate a suite of responses for protection against damage caused by UV-B radiation, with plants from high elevations or low latitudes generally displaying greater adaptation or tolerance to UV-B radiation. In an effort to explore the relationships between plant lignin levels and composition, the origin of growth elevation, and the hierarchical synthesis of UV-screening compounds, a collection of natural variants as well as transgenic Populus spp. were examined for sensitivity or acclimation to UV-B radiation under greenhouse and laboratory conditions. Noninvasive, ecophysiological measurements using epidermal transmittance and chlorophyll fluorescence as well as metabolite measurements using UPLC-MS generally revealed that the synthesis of anthocyanins, flavonoids, and lignin precursors are increased in Populus upon moderate to high UV-B treatment. However, poplar plants with genetic modifications that affect lignin biosynthesis, or natural variants with altered lignin levels and compositions, displayed complex changes in phenylpropanoid metabolites. A balance between elevated metabolic precursors to protective phenylpropanoids and increased biosynthesis of these anthocyanins, flavonoids, and lignin is proposed to play a role in the acclimation of Populus to UV-B radiation and may provide a useful tool in engineering plants as improved bioenergy feedstocks.

Multicolor plate reader fluorescence calibration.

Plate readers are commonly used to measure cell growth and fluorescence, yet the utility and reproducibility of plate reader data is limited by the fact that it is typically reported in arbitrary or relative units. We have previously established a robust serial dilution protocol for calibration of plate reader measurements of absorbance to estimated bacterial cell count and for green fluorescence from proteins expressed in bacterial cells to molecules of equivalent fluorescein. We now extend these protocols to calibration of red fluorescence to the sulforhodamine-101 fluorescent dye and blue fluorescence to Cascade Blue. Evaluating calibration efficacy via an interlaboratory study, we find that these calibrants do indeed provide comparable precision to the prior calibrants and that they enable effective cross-laboratory comparison of measurements of red and blue fluorescence from proteins expressed in bacterial cells.

Gene identification in black cohosh (Actaea racemosa L.): expressed sequence tag profiling and genetic screening yields candidate genes for production of bioactive secondary metabolites.

Black cohosh (Actaea racemosa L., syn. Cimicifuga racemosa, Nutt., Ranunculaceae) is a popular herb used for relieving menopausal discomforts. A variety of secondary metabolites, including triterpenoids, phenolic dimers, and serotonin derivatives have been associated with its biological activity, but the genes and metabolic pathways as well as the tissue distribution of their production in this plant are unknown. A gene discovery effort was initiated in A. racemosa by partial sequencing of cDNA libraries constructed from young leaf, rhizome, and root tissues. In total, 2,066 expressed sequence tags (ESTs) were assembled into 1,590 unique genes (unigenes). Most of the unigenes were predicted to encode primary metabolism genes, but about 70 were identified as putative secondary metabolism genes. Several of these candidates were analyzed further and full-length cDNA and genomic sequences for a putative 2,3 oxidosqualene cyclase (CAS1) and two BAHD-type acyltransferases (ACT1 and HCT1) were obtained. Homology-based PCR screening for the central gene in plant serotonin biosynthesis, tryptophan decarboxylase (TDC), identified two TDC-related sequences in A. racemosa. CAS1, ACT1, and HCT1 were expressed in most plant tissues, whereas expression of TDC genes was detected only sporadically in immature flower heads and some very young leaf tissues. The cDNA libraries described and assorted genes identified provide initial insight into gene content and diversity in black cohosh, and provide tools and resources for detailed investigations of secondary metabolite genes and enzymes in this important medicinal plant.

Structure of PqsD, a Pseudomonas quinolone signal biosynthetic enzyme, in complex with anthranilate.

Pseudomonas quinolone signal (PQS), 2-heptyl-3-hydroxy-4-quinolone, is an intercellular alkyl quinolone signaling molecule produced by the opportunistic pathogen Pseudomonas aeruginosa. Alkyl quinolone signaling is an atypical system that, in P. aeruginosa, controls the expression of numerous virulence factors. PQS is synthesized from the tryptophan pathway intermediate, anthranilate, which is derived either from the kynurenine pathway or from an alkyl quinolone specific anthranilate synthase encoded by phnAB. Anthranilate is converted to PQS by the enzymes encoded by the pqsABCDE operon and pqsH. PqsA forms an activated anthraniloyl-CoA thioester that shuttles anthranilate to the PqsD active site where it is transferred to Cys112 of PqsD. In the only biochemically characterized reaction, a condensation then occurs between anthraniloyl-PqsD and malonyl-CoA or malonyl-ACP, a second PqsD substrate, forming 2,4-dihydroxyquinoline (DHQ). The role PqsD plays in the biosynthesis of other alkyl quinolones, such as PQS, is unclear, though it has been reported to be required for their production. No evidence exists that DHQ is a PQS precursor, however. Here we present a structural and biophysical characterization of PqsD that includes several crystal structures of the enzyme, including that of the PqsD-anthranilate covalent intermediate and the inactive Cys112Ala active site mutant in complex with anthranilate. The structure reveals that PqsD is structurally similar to the FabH and chalcone synthase families of fatty acid and polyketide synthases. The crystallographic asymmetric unit contains a PqsD dimer. The PqsD monomer is composed of two nearly identical approximately 170-residue alphabetaalphabetaalpha domains. The structures show anthranilate-liganded Cys112 is positioned deep in the protein interior at the bottom of an approximately 15 A long channel while a second anthraniloyl-CoA molecule is waiting in the cleft leading to the protein surface. Cys112, His257, and Asn287 form the FabH-like catalytic triad of PqsD. The C112A mutant is inactive, although it still reversibly binds anthraniloyl-CoA. The covalent complex between anthranilate and Cys112 clearly illuminates the orientation of key elements of the PqsD catalytic machinery and represents a snapshot of a key point in the catalytic cycle.

Structural Insights into the Inhibitory Mechanism of an Antibody against B7-H6, a Stress-Induced Cellular Ligand for the Natural Killer Cell Receptor NKp30.

Antibodies have been shown to block signaling through cell surface receptors using several mechanisms. The two most common are binding to the ligand-binding site of the receptor and, conversely, binding to the receptor-binding site of the ligand. Here, we investigated the inhibitory mechanism of an antibody (17B1.3) against human B7-H6, a stress-induced cellular ligand for the natural killer (NK) cell receptor NKp30. Binding of this antibody to B7-H6, a transmembrane protein expressed on tumor and other stressed cells, but not on normal cells, prevents NK cell activation via NKp30. We determined the crystal structure of antibody 17B1.3 in complex with the ectodomain of B7-H6 to 2.5Å resolution. Surprisingly, 17B1.3 binds to a site on B7-H6 that is completely distinct from the binding site for NKp30, such that 17B1.3 does not block the NKp30-B7-H6 interaction. We then asked whether 17B1.3 prevents signaling by binding to a putative site for B7-H6 dimerization. However, structure-based mutations designed to disrupt potential B7-H6 dimerization through this site did not diminish NKp30-mediated cell activation. We conclude that the bulky 17B1.3 antibody most likely acts by sterically interfering with close cell-cell contacts at the NK cell-target cell interface that are required for NK cell activation. A similar inhibitory mechanism may apply to other antibodies, including therapeutic antibodies that block signaling through cell surface receptors whose ligands are also cell surface proteins.

NMR structure of HI0004, a putative essential gene product from Haemophilus influenzae, and comparison with the X-ray structure of an Aquifex aeolicus homolog.

The solution structure of the 154-residue conserved hypothetical protein HI0004 has been determined using multidimensional heteronuclear NMR spectroscopy. HI0004 has sequence homologs in many organisms ranging from bacteria to humans and is believed to be essential in Haemophilus influenzae, although an exact function has yet to be defined. It has a alpha-beta-alpha sandwich architecture consisting of a central four-stranded beta-sheet with the alpha2-helix packed against one side of the beta-sheet and four alpha-helices (alpha1, alpha3, alpha4, alpha5) on the other side. There is structural homology with the eukaryotic matrix metalloproteases (MMPs), but little sequence similarity except for a conserved region containing three histidines that appears in both the MMPs and throughout the HI0004 family of proteins. The solution structure of HI0004 is compared with the X-ray structure of an Aquifex aeolicus homolog, AQ_1354, which has 36% sequence identity over 148 residues. Despite this level of sequence homology, significant differences exist between the two structures. These differences are described along with possible functional implications of the structures.

Development of a Model Protein Interaction Pair as a Benchmarking Tool for the Quantitative Analysis of 2-Site Protein-Protein Interactions.

A significant challenge in the molecular interaction field is to accurately determine the stoichiometry and stepwise binding affinity constants for macromolecules having >1 binding site. The mission of the Molecular Interactions Research Group (MIRG) of the Association of Biomolecular Resource Facilities (ABRF) is to show how biophysical technologies are used to quantitatively characterize molecular interactions, and to educate the ABRF members and scientific community on the utility and limitations of core technologies [such as biosensor, microcalorimetry, or analytic ultracentrifugation (AUC)]. In the present work, the MIRG has developed a robust model protein interaction pair consisting of a bivalent variant of the Bacillus amyloliquefaciens extracellular RNase barnase and a variant of its natural monovalent intracellular inhibitor protein barstar. It is demonstrated that this system can serve as a benchmarking tool for the quantitative analysis of 2-site protein-protein interactions. The protein interaction pair enables determination of precise binding constants for the barstar protein binding to 2 distinct sites on the bivalent barnase binding partner (termed binase), where the 2 binding sites were engineered to possess affinities that differed by 2 orders of magnitude. Multiple MIRG laboratories characterized the interaction using isothermal titration calorimetry (ITC), AUC, and surface plasmon resonance (SPR) methods to evaluate the feasibility of the system as a benchmarking model. Although general agreement was seen for the binding constants measured using solution-based ITC and AUC approaches, weaker affinity was seen for surface-based method SPR, with protein immobilization likely affecting affinity. An analysis of the results from multiple MIRG laboratories suggests that the bivalent barnase-barstar system is a suitable model for benchmarking new approaches for the quantitative characterization of complex biomolecular interactions.

A multilaboratory comparison of calibration accuracy and the performance of external references in analytical ultracentrifugation.

Analytical ultracentrifugation (AUC) is a first principles based method to determine absolute sedimentation coefficients and buoyant molar masses of macromolecules and their complexes, reporting on their size and shape in free solution. The purpose of this multi-laboratory study was to establish the precision and accuracy of basic data dimensions in AUC and validate previously proposed calibration techniques. Three kits of AUC cell assemblies containing radial and temperature calibration tools and a bovine serum albumin (BSA) reference sample were shared among 67 laboratories, generating 129 comprehensive data sets. These allowed for an assessment of many parameters of instrument performance, including accuracy of the reported scan time after the start of centrifugation, the accuracy of the temperature calibration, and the accuracy of the radial magnification. The range of sedimentation coefficients obtained for BSA monomer in different instruments and using different optical systems was from 3.655 S to 4.949 S, with a mean and standard deviation of (4.304 ± 0.188) S (4.4%). After the combined application of correction factors derived from the external calibration references for elapsed time, scan velocity, temperature, and radial magnification, the range of s-values was reduced 7-fold with a mean of 4.325 S and a 6-fold reduced standard deviation of ± 0.030 S (0.7%). In addition, the large data set provided an opportunity to determine the instrument-to-instrument variation of the absolute radial positions reported in the scan files, the precision of photometric or refractometric signal magnitudes, and the precision of the calculated apparent molar mass of BSA monomer and the fraction of BSA dimers. These results highlight the necessity and effectiveness of independent calibration of basic AUC data dimensions for reliable quantitative studies.

From structure to function: YrbI from Haemophilus influenzae (HI1679) is a phosphatase.

The crystal structure of the YrbI protein from Haemophilus influenzae (HI1679) was determined at a 1.67-A resolution. The function of the protein had not been assigned previously, and it is annotated as hypothetical in sequence databases. The protein exhibits the alpha/beta-hydrolase fold (also termed the Rossmann fold) and resembles most closely the fold of the L-2-haloacid dehalogenase (HAD) superfamily. Following this observation, a detailed sequence analysis revealed remote homology to two members of the HAD superfamily, the P-domain of Ca(2+) ATPase and phosphoserine phosphatase. The 19-kDa chains of HI1679 form a tetramer both in solution and in the crystalline form. The four monomers are arranged in a ring such that four beta-hairpin loops, each inserted after the first beta-strand of the core alpha/beta-fold, form an eight-stranded barrel at the center of the assembly. Four active sites are located at the subunit interfaces. Each active site is occupied by a cobalt ion, a metal used for crystallization. The cobalt is octahedrally coordinated to two aspartate side-chains, a backbone oxygen, and three solvent molecules, indicating that the physiological metal may be magnesium. HI1679 hydrolyzes a number of phosphates, including 6-phosphogluconate and phosphotyrosine, suggesting that it functions as a phosphatase in vivo. The physiological substrate is yet to be identified; however the location of the gene on the yrb operon suggests involvement in sugar metabolism.

Crystal structure of the YjeE protein from Haemophilus influenzae: a putative Atpase involved in cell wall synthesis.

A hypothetical protein encoded by the gene YjeE of Haemophilus influenzae was selected as part of a structural genomics project for X-ray analysis to assist with the functional assignment. The protein is considered essential to bacteria because the gene is present in virtually all bacterial genomes but not in those of archaea or eukaryotes. The amino acid sequence shows no homology to other proteins except for the presence of the Walker A motif G-X-X-X-X-G-K-T that indicates the possibility of a nucleotide-binding protein. The YjeE protein was cloned, expressed, and the crystal structure determined by the MAD method at 1.7-A resolution. The protein has a nucleotide-binding fold with a four-stranded parallel beta-sheet flanked by antiparallel beta-strands on each side. The topology of the beta-sheet is unique among P-loop proteins and has features of different families of enzymes. Crystallization of YjeE in the presence of ATP and Mg2+ resulted in the structure with ADP bound in the P-loop. The ATPase activity of YjeE was confirmed by kinetic measurements. The distribution of conserved residues suggests that the protein may work as a "molecular switch" triggered by ATP hydrolysis. The phylogenetic pattern of YjeE suggests its involvement in cell wall biosynthesis.

Structure of the YibK methyltransferase from Haemophilus influenzae (HI0766): a cofactor bound at a site formed by a knot.

The crystal structures of YibK from Haemophilus influenzae (HI0766) have been determined with and without bound cofactor product S-adenosylhomocysteine (AdoHcy) at 1.7 and 2.0 A resolution, respectively. The molecule adopts an alpha/beta fold, with a topology that differs from that of the classical methyltransferases. Most notably, HI0766 contains a striking knot that forms the binding crevice for the cofactor. The knot formation is correlated with an alternative arrangement of the secondary structure units compared with the classical methyltransferases. Two loop regions undergo conformational changes upon AdoHcy binding. In contrast to the extended conformation of the cofactor seen in the classical methyltransferase structures, AdoHcy binds to HI0766 in a bent conformation. HI0766 and its close sequence relatives are all shorter versions of the more remotely related rRNA/tRNA methyltransferases of the spoU sequence family. We propose that the spoU sequence family contains the same core domain for cofactor binding as HI0766 but has an additional domain for substrate binding. The substrate-binding domain is absent in HI0766 sequence family and may be provided by another Haemophilus influenzae partner protein, which is yet to be identified.

Assisting functional assignment for hypothetical Heamophilus influenzae gene products through structural genomics.

The three-dimensional structures of Haemophilus influenzae proteins whose biological functions are unknown are being determined as part of a structural genomics project to ask whether structural information can assist in assigning the functions of proteins. The structures of the hypothetical proteins are being used to guide further studies and narrow the field of such studies for ultimately determining protein function. An outline of the structural genomics methodological approach is provided along with summaries of a number of completed and in progress crystallographic and NMR structure determinations. With more than twenty-five structures determined at this point and with many more in various stages of completion, the results are encouraging in that some level of functional understanding can be deduced from experimentally solved structures. In addition to aiding in functional assignment, this effort is identifying a number of possible new targets for drug development.

Thermodynamics of reactions catalyzed by PABA synthase.

Microcalorimetry and high-performance liquid chromatography (HPLC) have been used to conduct a thermodynamic investigation of reactions catalyzed by PABA synthase, the enzyme located at the first step in the shikimic acid metabolic pathway leading from chorismate to 4-aminobenzoate (PABA). The overall biochemical reaction catalyzed by the PabB and PabC components of PABA synthase is: chorismate(aq)+ammonia(aq)=4-aminobenzoate(aq)+pyruvate(aq)+H(2)O(l). This reaction can be divided into two partial reactions involving the intermediate 4-amino-4-deoxychorismate (ADC): chorismate(aq)+ammonia(aq)=ADC(aq)+H(2)O(l) and ADC(aq)=4-aminobenzoate(aq)+pyruvate(aq). Microcalorimetric measurements were performed on all three of these reactions at a temperature of 298.15 K and pH values in the range 8.72-8.77. Equilibrium measurements were performed on the first partial (ADC synthase) reaction at T=298.15 K and at pH=8.78. The saturation molality of 4-aminobenzoate(cr) in water is (0.00382+/-0.0004) mol kg(-1) at T=298.15 K. The results of the equilibrium and calorimetric measurements were analyzed in terms of a chemical equilibrium model that accounts for the multiplicity of ionic states of the reactants and products. These calculations gave thermodynamic quantities at the temperature 298.15 K and an ionic strength of zero for chemical reference reactions involving specific ionic forms. For the reaction: chorismate(2-)(aq)+NH(4)(+)(aq)=ADC(-)(aq)+H(2)O(l), K=(10.8+/-4.2) and Delta(r)H(m)(o)=-(35+/-15) kJ mol(-1). For the reaction: ADC(-)(aq)=4-aminobenzoate(-)(aq)+pyruvate(-)(aq)+H(+)(aq), Delta(r)H(m)(o)=-(139+/-23) kJ mol(-1). For the reaction: chorismate(2-)(aq)+NH(4)(+)(aq)=4-aminobenzoate(-)(aq)+pyruvate(-)(aq)+H(2)O(l)+H(+)(aq), Delta(r)H(m)(o)=-(174+/-6) kJ mol(-1). Thermodynamic cycle calculations were used to calculate thermodynamic quantities for three additional reactions that utilize L-glutamine rather than ammonia and that are pertinent to this branch point of the shikimic acid pathway. The quantities obtained in this study permit the calculation of the position of equilibrium of these reactions as a function of temperature, pH, and ionic strength. Values of the apparent equilibrium constants and the standard transformed Gibbs energy changes Delta(r)G'(m)(o) under approximately physiological conditions are given.

Systems approaches to unraveling plant metabolism: identifying biosynthetic genes of secondary metabolic pathways.

The diversity of useful compounds produced by plant secondary metabolism has stimulated broad systems biology approaches to identify the genes involved in their biosynthesis. Systems biology studies in non-model plants pose interesting but addressable challenges, and have been greatly facilitated by the ability to grow and maintain plants, develop laboratory culture systems, and profile key metabolites in order to identify critical genes involved their biosynthesis. In this chapter we describe a suite of approaches that have been useful in Actaea racemosa (L.; syn. Cimicifuga racemosa, Nutt., black coshosh), a non-model medicinal plant with no genome sequence and little horticultural information available, that have led to the development of initial gene-metabolite relationships for the production of several bioactive metabolites in this multicomponent botanical therapeutic, and that can be readily applied to a wide variety of under-characterized medicinal plants.

Calcium antagonists: a ready prescription for treating infectious diseases?

Emergence of new and medically resistant pathogenic microbes continues to escalate toward worldwide public health, wild habitat, and commercial crop and livestock catastrophes. Attempts at solving this problem with sophisticated modern biotechnologies, such as smart vaccines and microbicidal and microbistatic drugs that precisely target parasitic bacteria, fungi, and protozoa, remain promising without major clinical and industrial successes. However, discovery of a more immediate, broad spectrum prophylaxis beyond conventional epidemiological approaches might take no longer than the time required to fill a prescription at your neighborhood pharmacy. Findings from a growing body of research suggest calcium antagonists, long approved and marketed for various human cardiovascular and neurological indications, may produce safe, efficacious antimicrobial effects. As a general category of drugs, calcium antagonists include compounds that disrupt passage of Ca(2+) molecules across cell membranes and walls, sequestration and mobilization of free intracellular Ca(2+), and downstream binding proteins and sensors of Ca(2+)-dependent regulatory pathways important for proper cell function. Administration of calcium antagonists alone at current therapeutically relevant doses and schedules, or with synergistic compounds and additional antimicrobial medications, figures to enhance host immunoprotection by directly altering pathogen infection sequences, life cycles, homeostasis, antibiotic tolerances, and numerous other infective, survival, and reproductive processes. Short of being miracle drugs, calcium antagonists are welcome old drugs with new tricks capable of controlling some of the most virulent and pervasive global infectious diseases of plants, animals, and humans, including Chagas' disease, malaria, and tuberculosis.

ABRF-MIRG benchmark study: molecular interactions in a three-component system.

Protein-protein interactions identified through high-throughput proteomics efforts continue to advance our understanding of the protein interactome. In addition to highly specific protein-protein interactions, it is becoming increasingly more common for yeast two-hybrid, pull-down assays, and other proteomics techniques to identify multiple protein ligands that bind to the same target protein. A resulting challenge is to accurately characterize the assembly of these multiprotein complexes and the competition among multiple protein ligands for a given target. The Association of Biomolecular Resource Facilities-Molecular Interactions Research Group recently conducted a benchmark study to assess participants' ability to correctly describe the interactions between two protein ligands and their target protein using primarily biosensor technologies, such as surface plasmon resonance. Participants were provided with microgram quantities of three proteins (A, B, and C) and asked to determine if a ternary A-B-C complex can form or if protein-B and protein-C bind competitively to protein-A. This article will summarize the experimental approaches taken by participants to characterize the molecular interactions, the interpretation of the data, and the results obtained using different biosensor instruments.

Some new speculative ideas about the "behavioral homeostasis theory" as to how the simple learned behaviors of habituation and sensitization improve organism survival throughout phylogeny.

This paper explores further the "behavioral homeostasis theory" (BHT) regarding the evolutionary significance for organism survival of the two simple non-associative rapidly learned behaviors of habituation and sensitization. The BHT postulates that the evolutionary function of habituation and sensitization throughout phylogeny is to rapidly maximize an organism's overall readiness to cope with new stimuli and to minimize unnecessary energy expenditure. These behaviors have survived with remarkable similarity throughout phylogeny from aneural protozoa to humans. The concept of "behavioral homeostasis" emphasizes that the homeostatic process is more than just maintaining internal equilibrium in the face of changing internal and external conditions. It emphasizes the rapid internal and external effector system changes that occur to optimize organism readiness to cope with any new external stimulus situation. Truly life-threatening stimuli elicit instinctive behavior such as fight, flee, or hide. If the stimulus is not life-threatening, the organism rapidly learns to adjust to an appropriate level of overall responsiveness over stimulus repetitions. The rapid asymptotic level approached by those who decrease their overall responsiveness to the second stimulus (habituaters) and those who increase their overall responsiveness to an identical second stimulus (sensitizers) not only optimizes readiness to cope with any new stimulus situation but also reduces unnecessary energy expenditure. This paper is based on a retrospective analysis of data from 4 effector system responses to eight repetitive tone stimuli in adult human males. The effector systems include the galvanic skin response, finger pulse volume, muscle frontalis and heart rate. The new information provides the basis for further exploration of the BHT including new predictions and proposed relatively simple experiments to test them.