534 research outputs found
ConoServer: updated content, knowledge, and discovery tools in the conopeptide database
ConoServer (http://www.conoserver.org) is a database specializing in the sequences and structures of conopeptides, which are toxins expressed by marine cone snails. Cone snails are carnivorous gastropods, which hunt their prey using a cocktail of toxins that potently subvert nervous system function. The ability of these toxins to specifically target receptors, channels and transporters of the nervous system has attracted considerable interest for their use in physiological research and as drug leads. Since the founding publication on ConoServer in 2008, the number of entries in the database has nearly doubled, the interface has been redesigned and new annotations have been added, including a more detailed description of cone snail species, biological activity measurements and information regarding the identification of each sequence. Automatically updated statistics on classification schemes, three-dimensional structures, conopeptide-bearing species and endoplasmic reticulum signal sequence conservation trends, provide a convenient overview of current knowledge on conopeptides. Transcriptomics and proteomics have began generating massive numbers of new conopeptide sequences, and two dedicated tools have been recently implemented in ConoServer to standardize the analysis of conopeptide precursor sequences and to help in the identification by mass spectrometry of toxins whose sequences were predicted at the nucleic acid level
Splitting Proofs for Interpolation
We study interpolant extraction from local first-order refutations. We
present a new theoretical perspective on interpolation based on clearly
separating the condition on logical strength of the formula from the
requirement on the com- mon signature. This allows us to highlight the space of
all interpolants that can be extracted from a refutation as a space of simple
choices on how to split the refuta- tion into two parts. We use this new
insight to develop an algorithm for extracting interpolants which are linear in
the size of the input refutation and can be further optimized using metrics
such as number of non-logical symbols or quantifiers. We implemented the new
algorithm in first-order theorem prover VAMPIRE and evaluated it on a large
number of examples coming from the first-order proving community. Our
experiments give practical evidence that our work improves the state-of-the-art
in first-order interpolation.Comment: 26th Conference on Automated Deduction, 201
A Reduction from Unbounded Linear Mixed Arithmetic Problems into Bounded Problems
We present a combination of the Mixed-Echelon-Hermite transformation and the
Double-Bounded Reduction for systems of linear mixed arithmetic that preserve
satisfiability and can be computed in polynomial time. Together, the two
transformations turn any system of linear mixed constraints into a bounded
system, i.e., a system for which termination can be achieved easily. Existing
approaches for linear mixed arithmetic, e.g., branch-and-bound and cuts from
proofs, only explore a finite search space after application of our two
transformations. Instead of generating a priori bounds for the variables, e.g.,
as suggested by Papadimitriou, unbounded variables are eliminated through the
two transformations. The transformations orient themselves on the structure of
an input system instead of computing a priori (over-)approximations out of the
available constants. Experiments provide further evidence to the efficiency of
the transformations in practice. We also present a polynomial method for
converting certificates of (un)satisfiability from the transformed to the
original system
Synthesis, pharmacological and structural characterization of novel conopressins from Conus miliaris
Cone snails produce a fast-acting and often paralyzing venom, largely dominated by
disulfide-rich conotoxins targeting ion channels. Although disulfide-poor conopeptides are usually
minor components of cone snail venoms, their ability to target key membrane receptors such as
GPCRs make them highly valuable as drug lead compounds. From the venom gland transcriptome
of Conus miliaris, we report here on the discovery and characterization of two conopressins, which are
nonapeptide ligands of the vasopressin/oxytocin receptor family. These novel sequence variants show
unusual features, including a charge inversion at the critical position 8, with an aspartate instead
of a highly conserved lysine or arginine residue. Both the amidated and acid C-terminal analogues
were synthesized, followed by pharmacological characterization on human and zebrafish receptors
and structural investigation by NMR. Whereas conopressin-M1 showed weak and only partial
agonist activity at hV1bR (amidated form only) and ZFV1a1R (both amidated and acid form), both
conopressin-M2 analogues acted as full agonists at the ZFV2 receptor with low micromolar a�nity.
Together with the NMR structures of amidated conopressins-M1, -M2 and -G, this study provides
novel structure-activity relationship information that may help in the design of more selective ligands
Periplasmic Expression of 4/7 α-Conotoxin TxIA Analogs in E. coli Favors Ribbon Isomer Formation – Suggestion of a Binding Mode at the α7 nAChR
Peptides derived from animal venoms provide important research tools for biochemical and pharmacological characterization of receptors, ion channels, and transporters. Some venom peptides have been developed into drugs (such as the synthetic omega-conotoxin MVIIA, ziconotide) and several are currently undergoing clinical trials for various clinical indications. Challenges in the development of peptides include their usually limited supply from natural sources, cost-intensive chemical synthesis, and potentially complicated stereoselective disulfide-bond formation in the case of disulfide-rich peptides. In particular, if extended structure-function analysis is performed or incorporation of stable isotopes for NMR studies is required, the comparatively low yields and high costs of synthesized peptides might constitute a limiting factor. Here we investigated the expression of the 4/7 alpha-conotoxin TxIA, a potent blocker at alpha 3 beta 2 and alpha 7 nicotinic acetylcholine receptors (nAChRs), and three analogs in the form of maltose binding protein fusion proteins in Escherichia coli. Upon purification via nickel affinity chromatography and release of the toxins by protease cleavage, HPLC analysis revealed one major peak with the correct mass for all peptides. The final yield was 1-2 mg of recombinant peptide per liter of bacterial culture. Two-electrode voltage clamp analysis on oocyte-expressed nAChR subtypes demonstrated the functionality of these peptides but also revealed a 30 to 100-fold potency decrease of expressed TxIA compared to chemically synthesized TxIA. NMR spectroscopy analysis of TxIA and two of its analogs confirmed that the decreased activity was due to an alternative disulfide linkage rather than the missing C-terminal amidation, a post-translational modification that is common in alpha-conotoxins. All peptides preferentially formed in the ribbon conformation rather than the native globular conformation. Interestingly, in the case of the alpha 7 nAChR, but not the alpha 3 beta 2 subtype, the loss of potency could be rescued by an R5D substitution. In conclusion, we demonstrate efficient expression of functional but alternatively folded ribbon TxIA variants in E. coli and provide the first structure-function analysis for a ribbon 4/7-alpha-conotoxin at alpha 7 and alpha 3 beta 2 nAChRs. Computational analysis based on these data provide evidence for a ribbon alpha-conotoxin binding mode that might be exploited to design ligands with optimized selectivity
An iterative approach to precondition inference using constrained Horn clauses
We present a method for automatic inference of conditions on the initial
states of a program that guarantee that the safety assertions in the program
are not violated. Constrained Horn clauses (CHCs) are used to model the program
and assertions in a uniform way, and we use standard abstract interpretations
to derive an over-approximation of the set of unsafe initial states. The
precondition then is the constraint corresponding to the complement of that
set, under-approximating the set of safe initial states. This idea of
complementation is not new, but previous attempts to exploit it have suffered
from the loss of precision. Here we develop an iterative specialisation
algorithm to give more precise, and in some cases optimal safety conditions.
The algorithm combines existing transformations, namely constraint
specialisation, partial evaluation and a trace elimination transformation. The
last two of these transformations perform polyvariant specialisation, leading
to disjunctive constraints which improve precision. The algorithm is
implemented and tested on a benchmark suite of programs from the literature in
precondition inference and software verification competitions.Comment: Paper presented at the 34nd International Conference on Logic
Programming (ICLP 2018), Oxford, UK, July 14 to July 17, 2018 18 pages, LaTe
Formalising the Continuous/Discrete Modeling Step
Formally capturing the transition from a continuous model to a discrete model
is investigated using model based refinement techniques. A very simple model
for stopping (eg. of a train) is developed in both the continuous and discrete
domains. The difference between the two is quantified using generic results
from ODE theory, and these estimates can be compared with the exact solutions.
Such results do not fit well into a conventional model based refinement
framework; however they can be accommodated into a model based retrenchment.
The retrenchment is described, and the way it can interface to refinement
development on both the continuous and discrete sides is outlined. The approach
is compared to what can be achieved using hybrid systems techniques.Comment: In Proceedings Refine 2011, arXiv:1106.348
A Survey of Satisfiability Modulo Theory
Satisfiability modulo theory (SMT) consists in testing the satisfiability of
first-order formulas over linear integer or real arithmetic, or other theories.
In this survey, we explain the combination of propositional satisfiability and
decision procedures for conjunctions known as DPLL(T), and the alternative
"natural domain" approaches. We also cover quantifiers, Craig interpolants,
polynomial arithmetic, and how SMT solvers are used in automated software
analysis.Comment: Computer Algebra in Scientific Computing, Sep 2016, Bucharest,
Romania. 201
Applying SMT Solvers to the Test Template Framework
The Test Template Framework (TTF) is a model-based testing method for the Z
notation. In the TTF, test cases are generated from test specifications, which
are predicates written in Z. In turn, the Z notation is based on first-order
logic with equality and Zermelo-Fraenkel set theory. In this way, a test case
is a witness satisfying a formula in that theory. Satisfiability Modulo Theory
(SMT) solvers are software tools that decide the satisfiability of arbitrary
formulas in a large number of built-in logical theories and their combination.
In this paper, we present the first results of applying two SMT solvers, Yices
and CVC3, as the engines to find test cases from TTF's test specifications. In
doing so, shallow embeddings of a significant portion of the Z notation into
the input languages of Yices and CVC3 are provided, given that they do not
directly support Zermelo-Fraenkel set theory as defined in Z. Finally, the
results of applying these embeddings to a number of test specifications of
eight cases studies are analysed.Comment: In Proceedings MBT 2012, arXiv:1202.582
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