Temperature-Dependent Fold-Switching Mechanism of the Circadian Clock Protein KaiB.

The oscillator of the cyanobacterial circadian clock relies on the ability of the KaiB protein to switch reversibly between a stable ground-state fold (gsKaiB) and an unstable fold-switched fold (fsKaiB). Rare fold-switching events by KaiB provide a critical delay in the negative feedback loop of this post-translational oscillator. In this study, we experimentally and computationally investigate the temperature dependence of fold switching and its mechanism. We demonstrate that the stability of gsKaiB increases with temperature compared to fsKaiB and that the Q10 value for the gsKaiB → fsKaiB transition is nearly three times smaller than that for the reverse transition. Simulations and native-state hydrogen-deuterium exchange NMR experiments suggest that fold switching can involve both subglobally and near-globally unfolded intermediates. The simulations predict that the transition state for fold switching coincides with isomerization of conserved prolines in the most rapidly exchanging region, and we confirm experimentally that proline isomerization is a rate-limiting step for fold switching. We explore the implications of our results for temperature compensation, a hallmark of circadian clocks, through a kinetic model.

Protocols for in vitro reconstitution of the cyanobacterial circadian clock.

Circadian clocks are intracellular systems that orchestrate metabolic processes in anticipation of sunrise and sunset by providing an internal representation of local time. Because the ~24-h metabolic rhythms they produce are important to health across diverse life forms there is growing interest in their mechanisms. However, mechanistic studies are challenging in vivo due to the complex, that is, poorly defined, milieu of live cells. Recently, we reconstituted the intact circadian clock of cyanobacteria in vitro. It oscillates autonomously and remains phase coherent for many days with a fluorescence-based readout that enables real-time observation of individual clock proteins and promoter DNA simultaneously under defined conditions without user intervention. We found that reproducibility of the reactions required strict adherence to the quality of each recombinant clock protein purified from Escherichia coli. Here, we provide protocols for preparing in vitro clock samples so that other labs can ask questions about how changing environments, like temperature, metabolites, and protein levels are reflected in the core oscillator and propagated to regulation of transcription, providing deeper mechanistic insights into clock biology.

Synchronization of the circadian clock to the environment tracked in real time.

The circadian system of the cyanobacterium Synechococcus elongatus PCC 7942 relies on a three-protein nanomachine (KaiA, KaiB, and KaiC) that undergoes an oscillatory phosphorylation cycle with a period of ~24 h. This core oscillator can be reconstituted in vitro and is used to study the molecular mechanisms of circadian timekeeping and entrainment. Previous studies showed that two key metabolic changes that occur in cells during the transition into darkness, changes in the ATP/ADP ratio and redox status of the quinone pool, are cues that entrain the circadian clock. By changing the ATP/ADP ratio or adding oxidized quinone, one can shift the phase of the phosphorylation cycle of the core oscillator in vitro. However, the in vitro oscillator cannot explain gene expression patterns because the simple mixture lacks the output components that connect the clock to genes. Recently, a high-throughput in vitro system termed the in vitro clock (IVC) that contains both the core oscillator and the output components was developed. Here, we used IVC reactions and performed massively parallel experiments to study entrainment, the synchronization of the clock with the environment, in the presence of output components. Our results indicate that the IVC better explains the in vivo clock-resetting phenotypes of wild-type and mutant strains and that the output components are deeply engaged with the core oscillator, affecting the way input signals entrain the core pacemaker. These findings blur the line between input and output pathways and support our previous demonstration that key output components are fundamental parts of the clock.

Coupling of distant ATPase domains in the circadian clock protein KaiC.

The AAA+ family member KaiC is the central pacemaker for circadian rhythms in the cyanobacterium Synechococcus elongatus. Composed of two hexameric rings of adenosine triphosphatase (ATPase) domains with tightly coupled activities, KaiC undergoes a cycle of autophosphorylation and autodephosphorylation on its C-terminal (CII) domain that restricts binding of clock proteins on its N-terminal (CI) domain to the evening. Here, we use cryogenic-electron microscopy to investigate how daytime and nighttime states of CII regulate KaiB binding on CI. We find that the CII hexamer is destabilized during the day but takes on a rigidified C2-symmetric state at night, concomitant with ring-ring compression. Residues at the CI-CII interface are required for phospho-dependent KaiB association, coupling ATPase activity on CI to cooperative KaiB recruitment. Together, these studies clarify a key step in the regulation of cyanobacterial circadian rhythms by KaiC phosphorylation.

A Night-Time Edge Site Intermediate in the Cyanobacterial Circadian Clock Identified by EPR Spectroscopy.

As the only circadian oscillator that can be reconstituted in vitro with its constituent proteins KaiA, KaiB, and KaiC using ATP as an energy source, the cyanobacterial circadian oscillator serves as a model system for detailed mechanistic studies of day-night transitions of circadian clocks in general. The day-to-night transition occurs when KaiB forms a night-time complex with KaiC to sequester KaiA, the latter of which interacts with KaiC during the day to promote KaiC autophosphorylation. However, how KaiB forms the complex with KaiC remains poorly understood, despite the available structures of KaiB bound to hexameric KaiC. It has been postulated that KaiB-KaiC binding is regulated by inter-KaiB cooperativity. Here, using spin labeling continuous-wave electron paramagnetic resonance spectroscopy, we identified and quantified two subpopulations of KaiC-bound KaiB, corresponding to the "bulk" and "edge" KaiBC sites in stoichiometric and substoichiometric KaiBiC6 complexes (i = 1-5). We provide kinetic evidence to support the intermediacy of the "edge" KaiBC sites as bridges and nucleation sites between free KaiB and the "bulk" KaiBC sites. Furthermore, we show that the relative abundance of "edge" and "bulk" sites is dependent on both KaiC phosphostate and KaiA, supporting the notion of phosphorylation-state controlled inter-KaiB cooperativity. Finally, we demonstrate that the interconversion between the two subpopulations of KaiC-bound KaiB is intimately linked to the KaiC phosphorylation cycle. These findings enrich our mechanistic understanding of the cyanobacterial clock and demonstrate the utility of EPR in elucidating circadian clock mechanisms.

Reconstitution of an intact clock reveals mechanisms of circadian timekeeping.

Circadian clocks control gene expression to provide an internal representation of local time. We report reconstitution of a complete cyanobacterial circadian clock in vitro, including the central oscillator, signal transduction pathways, downstream transcription factor, and promoter DNA. The entire system oscillates autonomously and remains phase coherent for many days with a fluorescence-based readout that enables real-time observation of each component simultaneously without user intervention. We identified the molecular basis for loss of cycling in an arrhythmic mutant and explored fundamental mechanisms of timekeeping in the cyanobacterial clock. We find that SasA, a circadian sensor histidine kinase associated with clock output, engages directly with KaiB on the KaiC hexamer to regulate period and amplitude of the central oscillator. SasA uses structural mimicry to cooperatively recruit the rare, fold-switched conformation of KaiB to the KaiC hexamer to form the nighttime repressive complex and enhance rhythmicity of the oscillator, particularly under limiting concentrations of KaiB. Thus, the expanded in vitro clock reveals previously unknown mechanisms by which the circadian system of cyanobacteria maintains the pace and rhythmicity under variable protein concentrations.

Real-Time In Vitro Fluorescence Anisotropy of the Cyanobacterial Circadian Clock.

Stochastic diffusion of a solution of fluorophores after photoselection reduces the polarization of emission, or fluorescence anisotropy. Because this randomization process is slower for larger molecules, fluorescence anisotropy is effective for measuring the kinetics of protein-binding events. Here, we describe how to use the technique to carry out real-time observations in vitro of the cyanobacterial circadian clock.

Bayesian modeling reveals metabolite-dependent ultrasensitivity in the cyanobacterial circadian clock.

Mathematical models can enable a predictive understanding of mechanism in cell biology by quantitatively describing complex networks of interactions, but such models are often poorly constrained by available data. Owing to its relative biochemical simplicity, the core circadian oscillator in Synechococcus elongatus has become a prototypical system for studying how collective dynamics emerge from molecular interactions. The oscillator consists of only three proteins, KaiA, KaiB, and KaiC, and near-24-h cycles of KaiC phosphorylation can be reconstituted in vitro. Here, we formulate a molecularly detailed but mechanistically naive model of the KaiA-KaiC subsystem and fit it directly to experimental data within a Bayesian parameter estimation framework. Analysis of the fits consistently reveals an ultrasensitive response for KaiC phosphorylation as a function of KaiA concentration, which we confirm experimentally. This ultrasensitivity primarily results from the differential affinity of KaiA for competing nucleotide-bound states of KaiC. We argue that the ultrasensitive stimulus-response relation likely plays an important role in metabolic compensation by suppressing premature phosphorylation at nighttime.

Monitoring Protein-Protein Interactions in the Cyanobacterial Circadian Clock in Real Time via Electron Paramagnetic Resonance Spectroscopy.

The cyanobacterial circadian clock in Synechococcus elongatus consists of three proteins, KaiA, KaiB, and KaiC. KaiA and KaiB rhythmically interact with KaiC to generate stable oscillations of KaiC phosphorylation with a period of 24 h. The observation of stable circadian oscillations when the three clock proteins are reconstituted and combined in vitro makes it an ideal system for understanding its underlying molecular mechanisms and circadian clocks in general. These oscillations were historically monitored in vitro by gel electrophoresis of reaction mixtures based on the differing electrophoretic mobilities between various phosphostates of KaiC. As the KaiC phospho-distribution represents only one facet of the oscillations, orthogonal tools are necessary to explore other interactions to generate a full description of the system. However, previous biochemical assays are discontinuous or qualitative. To circumvent these limitations, we developed a spin-labeled KaiB mutant that can differentiate KaiC-bound KaiB from free KaiB using continuous-wave electron paramagnetic resonance spectroscopy that is minimally sensitive to KaiA. Similar to wild-type (WT-KaiB), this labeled mutant, in combination with KaiA, sustains robust circadian rhythms of KaiC phosphorylation. This labeled mutant is hence a functional surrogate of WT-KaiB and thus participates in and reports on autonomous macroscopic circadian rhythms generated by mixtures that include KaiA, KaiC, and ATP. Quantitative kinetics could be extracted with improved precision and time resolution. We describe design principles, data analysis, and limitations of this quantitative binding assay and discuss future research necessary to overcome these challenges.

Real-Time In Vitro Fluorescence Anisotropy of the Cyanobacterial Circadian Clock.

Uniquely, the circadian clock of cyanobacteria can be reconstructed outside the complex milieu of live cells, greatly simplifying the investigation of a functioning biological chronometer. The core oscillator component is composed of only three proteins, KaiA, KaiB, and KaiC, and together with ATP they undergo waves of assembly and disassembly that drive phosphorylation rhythms in KaiC. Typically, the time points of these reactions are analyzed ex post facto by denaturing polyacrylamide gel electrophoresis, because this technique resolves the different states of phosphorylation of KaiC. Here, we describe a more sensitive method that allows real-time monitoring of the clock reaction. By labeling one of the clock proteins with a fluorophore, in this case KaiB, the in vitro clock reaction can be monitored by fluorescence anisotropy on the minutes time scale for weeks.

Structural basis of the day-night transition in a bacterial circadian clock.

Circadian clocks are ubiquitous timing systems that induce rhythms of biological activities in synchrony with night and day. In cyanobacteria, timing is generated by a posttranslational clock consisting of KaiA, KaiB, and KaiC proteins and a set of output signaling proteins, SasA and CikA, which transduce this rhythm to control gene expression. Here, we describe crystal and nuclear magnetic resonance structures of KaiB-KaiC,KaiA-KaiB-KaiC, and CikA-KaiB complexes. They reveal how the metamorphic properties of KaiB, a protein that adopts two distinct folds, and the post-adenosine triphosphate hydrolysis state of KaiC create a hub around which nighttime signaling events revolve, including inactivation of KaiA and reciprocal regulation of the mutually antagonistic signaling proteins, SasA and CikA.

An amino acid code for irregular and mixed protein packing.

To advance our understanding of protein tertiary structure, the development of the knob-socket model is completed in an analysis of the packing in irregular coil and turn secondary structure packing as well as between mixed secondary structure. The knob-socket model simplifies packing based on repeated patterns of two motifs: a three-residue socket for packing within secondary (2°) structure and a four-residue knob-socket for tertiary (3°) packing. For coil and turn secondary structure, knob-sockets allow identification of a correlation between amino acid composition and tertiary arrangements in space. Coil contributes almost as much as α-helices to tertiary packing. In irregular sockets, Gly, Pro, Asp, and Ser are favored, while in irregular knobs, the preference order is Arg, Asp, Pro, Asn, Thr, Leu, and Gly. Cys, His,Met, and Trp are not favored in either. In mixed packing, the knob amino acid preferences are a function of the socket that they are packing into, whereas the amino acid composition of the sockets does not depend on the secondary structure of the knob. A unique motif of a coil knob with an XYZ ß-sheet socket may potentially function to inhibit ß-sheet extension. In addition, analysis of the preferred crossing angles for strands within a ß-sheet and mixed α-helice/ß-sheet identifies canonical packing patterns useful in protein design. Lastly, the knob-socket model abstracts the complexity of protein tertiary structure into an intuitive packing surface topology map.

The effect of α-mating factor secretion signal mutations on recombinant protein expression in Pichia pastoris.

The methylotrophic yeast, Pichia pastoris, has been genetically engineered to produce many heterologous proteins for industrial and research purposes. In order to secrete proteins for easier purification from the extracellular medium, the coding sequence of recombinant proteins is initially fused to the Saccharomyces cerevisiae α-mating factor secretion signal leader. Extensive site-directed mutagenesis of the prepro-region of the α-mating factor secretion signal sequence was performed in order to determine the effects of various deletions and substitutions on expression. Though some mutations clearly dampened protein expression, deletion of amino acids 57-70, corresponding to the predicted 3rd alpha helix of α-mating factor secretion signal, increased secretion of reporter proteins horseradish peroxidase and lipase at least 50% in small-scale cultures. These findings raise the possibility that the secretory efficiency of the leader can be further enhanced in the future.

Understanding the general packing rearrangements required for successful template based modeling of protein structure from a CASP experiment.

As an alternative to the common template based protein structure prediction methods based on main-chain position, a novel side-chain centric approach has been developed. Together with a Bayesian loop modeling procedure and a combination scoring function, the Stone Soup algorithm was applied to the CASP9 set of template based modeling targets. Although the method did not generate as large of perturbations to the template structures as necessary, the analysis of the results gives unique insights into the differences in packing between the target structures and their templates. Considerable variation in packing is found between target and template structures even when the structures are close, and this variation is found due to 2 and 3 body packing interactions. Outside the inherent restrictions in packing representation of the PDB, the first steps in correctly defining those regions of variable packing have been mapped primarily to local interactions, as the packing at the secondary and tertiary structure are largely conserved. Of the scoring functions used, a loop scoring function based on water structure exhibited some promise for discrimination. These results present a clear structural path for further development of a side-chain centered approach to template based modeling.

An amino acid packing code for α-helical structure and protein design.

This work demonstrates that all packing in α-helices can be simplified to repetitive patterns of a single motif: the knob-socket. Using the precision of Voronoi Polyhedra/Delauney Tessellations to identify contacts, the knob-socket is a four-residue tetrahedral motif: a knob residue on one α-helix packs into the three-residue socket on another α-helix. The principle of the knob-socket model relates the packing between levels of protein structure: the intra-helical packing arrangements within secondary structure that permit inter-helix tertiary packing interactions. Within an α-helix, the three-residue sockets arrange residues into a uniform packing lattice. Inter-helix packing results from a definable pattern of interdigitated knob-socket motifs between two α-helices. Furthermore, the knob-socket model classifies three types of sockets: (1) free, favoring only intra-helical packing; (2) filled, favoring inter-helical interactions; and (3) non, disfavoring α-helical structure. The amino acid propensities in these three socket classes essentially represent an amino acid code for structure in α-helical packing. Using this code, we used a novel yet straightforward approach for the design of α-helical structure to validate the knob-socket model. Unique sequences for three peptides were created to produce a predicted amount of α-helical structure: mostly helical, some helical, and no helix. These three peptides were synthesized, and helical content was assessed using CD spectroscopy. The measured α-helicity of each peptide was consistent with the expected predictions. These results and analysis demonstrate that the knob-socket motif functions as the basic unit of packing and presents an intuitive tool to decipher the rules governing packing in protein structure.

Near-native protein loop sampling using nonparametric density estimation accommodating sparcity.

Unlike the core structural elements of a protein like regular secondary structure, template based modeling (TBM) has difficulty with loop regions due to their variability in sequence and structure as well as the sparse sampling from a limited number of homologous templates. We present a novel, knowledge-based method for loop sampling that leverages homologous torsion angle information to estimate a continuous joint backbone dihedral angle density at each loop position. The φ,ψ distributions are estimated via a Dirichlet process mixture of hidden Markov models (DPM-HMM). Models are quickly generated based on samples from these distributions and were enriched using an end-to-end distance filter. The performance of the DPM-HMM method was evaluated against a diverse test set in a leave-one-out approach. Candidates as low as 0.45 Å RMSD and with a worst case of 3.66 Å were produced. For the canonical loops like the immunoglobulin complementarity-determining regions (mean RMSD <2.0 Å), the DPM-HMM method performs as well or better than the best templates, demonstrating that our automated method recaptures these canonical loops without inclusion of any IgG specific terms or manual intervention. In cases with poor or few good templates (mean RMSD >7.0 Å), this sampling method produces a population of loop structures to around 3.66 Å for loops up to 17 residues. In a direct test of sampling to the Loopy algorithm, our method demonstrates the ability to sample nearer native structures for both the canonical CDRH1 and non-canonical CDRH3 loops. Lastly, in the realistic test conditions of the CASP9 experiment, successful application of DPM-HMM for 90 loops from 45 TBM targets shows the general applicability of our sampling method in loop modeling problem. These results demonstrate that our DPM-HMM produces an advantage by consistently sampling near native loop structure. The software used in this analysis is available for download at http://www.stat.tamu.edu/~dahl/software/cortorgles/.

Target-specific anti-fungal discovery by targeting Geotrichum candidum histidinol dehydrogenase: a hybrid approach.

This study describes a hybrid approach of screening substrate analogue inhibitors of histidinol dehydrogenase. Imidazole derivative library of approximately 400 compounds classified using Hierarchical cluster analysis, representative compounds of each class were tested in enzymatic assay and used for the development of quantitative structure-activity relationship models. Rest of the compounds in the library were screened using developed models and compounds predicted active were retested. 60% of the predicted compounds showed enzyme inhibition activity with IC(50) values ranged between 5.2 and 58.0 microm range and have fungistatic activity.

Selectivity-based QSAR approach for screening and evaluation of TRH analogs for TRH-R1 and TRH-R2 receptors subtypes.

Design and development of therapeutically useful CNS selective thyrotropin-releasing hormone (TRH) analogs acting on TRH-R2 receptor subtype, exerting weak or no TRH-R1-mediated TSH-releasing side effects has gained imagination of researchers in the recent past. The present study reports the development and implementation of a selectivity-based QSAR approach for screening selective agonists of TRH-R2 receptor subtype. The statistically significant predictive models were thoroughly validated using an external validation set whose activity was previously unknown. The model was able to predict preference for either of the receptor subtypes successfully.

Evaluation of Pseudomonas aeruginosa deacetylase LpxC inhibitory activity of dual PDE4-TNFalpha inhibitors: a multiscreening approach.

In this study, we have focused on the implication of a multiscreening approach in the evaluation of Pseudomonas aeruginosa deacetylase LpxC inhibitory activity of dual PDE4-TNFalpha inhibitors. A genetic function approximation (GFA) directed quantitative structure-activity relationship (QSAR) model was developed for LpxC inhibition on the basis of reported biological activity (Kline and Andersen, J. Med. Chem. 2002, 45, 3112-3129). Subsequently, reported PDE4-TNFalpha inhibitors (Klienman and Campbell, J. Med. Chem. 1998, 41, 266-270) were screened using the QSAR model. Whereby, the compounds were predicted to have equipotent activity with the most potent compound in reported LpxC inhibitor series. A docking analysis of these compounds carried out on the LpxC homology model corroborated the initial results. The compounds were then validated using surface electronic properties analysis and subjected to an adsorption, distribution, metabolism, excretion, and toxicity filter. Taken together, a multiscreening strategy was used to validate potential leads for LpxC inhibition.

Pharmacophoric features of Pseudomonas aeruginosa deacetylase LpxC inhibitors: an electronic and structural analysis.

Various electronic properties of structurally diverse synthetic LpxC inhibitors containing oxazoline, aroylserine and thiazoline rings were calculated and correlated with biological activity. These electronic features include the magnitude and locations of 3-dimensional molecular electrostatic potentials, hydrogen bond acceptor/donor density, lowest unoccupied molecular orbital, and highest occupied molecular orbital. Strong correlation of these stereo-electronic properties with LpxC inhibitory potency reveals the potential pharmacophoric features of specific LpxC inhibitors. Thus, these pharmacophoric features of LpxC inhibitors based on electronic and surface analysis could be successfully exploited for designing more potent LpxC inhibitors.