Matías R. Machado

SIRAH: A Structurally Unbiased Coarse-Grained Force Field for Proteins with Aqueous Solvation and Long-Range Electrostatics

Modeling of macromolecular structures and interactions represents an important challenge for computational biology, involving different time and length scales. However, this task can be facilitated through the use of coarse-grained (CG) models, which reduce the number of degrees of freedom and allow efficient exploration of complex conformational spaces. This article presents a new CG protein model named SIRAH, developed to work with explicit solvent and to capture sequence, temperature, and ionic strength effects in a topologically unbiased manner. SIRAH is implemented in GROMACS, and interactions are calculated using a standard pairwise Hamiltonian for classical molecular dynamics simulations. We present a set of simulations that test the capability of SIRAH to produce a qualitatively correct solvation on different amino acids, hydrophilic/hydrophobic interactions, and long-range electrostatic recognition leading to spontaneous association of unstructured peptides and stable structures of single polypeptides and protein-protein complexes.

Coarse-grained models of water

Coarse‐grained (CG) models for macromolecules have become a standard in the study of biological systems, overcoming limitations in size and time scales encountered by atomistic molecular dynamics simulations. Just as in any biomolecular ensemble, water in CG models plays a key role in mediating intermolecular and intramolecular interactions. However, owing to the highly nontrivial properties of water, important simplifications have been commonly used to treat solvation effects. Recent developments of CG models for water are overviewed, comparing some characteristic features and limitations.

FRET biosensor uncovers cAMP nano-domains at β-adrenergic targets that dictate precise tuning of cardiac contractility.

Compartmentalized cAMP/PKA signalling is now recognized as important for physiology and pathophysiology, yet a detailed understanding of the properties, regulation and function of local cAMP/PKA signals is lacking. Here we present a fluorescence resonance energy transfer (FRET)-based sensor, CUTie, which detects compartmentalized cAMP with unprecedented accuracy. CUTie, targeted to specific multiprotein complexes at discrete plasmalemmal, sarcoplasmic reticular and myofilament sites, reveals differential kinetics and amplitudes of localized cAMP signals. This nanoscopic heterogeneity of cAMP signals is necessary to optimize cardiac contractility upon adrenergic activation. At low adrenergic levels, and those mimicking heart failure, differential local cAMP responses are exacerbated, with near abolition of cAMP signalling at certain locations. This work provides tools and fundamental mechanistic insights into subcellular adrenergic signalling in normal and pathological cardiac function.

Regulation of signaling directionality revealed by 3D snapshots of a kinase:regulator complex in action

Two-component systems (TCS) are protein machineries that enable cells to respond to input signals. Histidine kinases (HK) are the sensory component, transferring information toward downstream response regulators (RR). HKs transfer phosphoryl groups to their specific RRs, but also dephosphorylate them, overall ensuring proper signaling. The mechanisms by which HKs discriminate between such disparate directions, are yet unknown. We now disclose crystal structures of the HK:RR complex DesK:DesR from Bacillus subtilis, comprising snapshots of the phosphotransfer and the dephosphorylation reactions. The HK dictates the reactional outcome through conformational rearrangements that include the reactive histidine. The phosphotransfer center is asymmetric, poised for dissociative nucleophilic substitution. The structural bases of HK phosphatase/phosphotransferase control are uncovered, and the unexpected discovery of a dissociative reactional center, sheds light on the evolution of TCS phosphotransfer reversibility. Our findings should be applicable to a broad range of signaling systems and instrumental in synthetic TCS rewiring. DOI: http://dx.doi.org/10.7554/eLife.21422.001

Exploring LacI–DNA Dynamics by Multiscale Simulations Using the SIRAH Force Field

The lac repressor protein (LacI) together with its target regulatory sequence are a common model for studying DNA looping and its implications on transcriptional control in bacteria. Owing to the molecular size of this system, standard all-atom (AA) simulations are prohibitive for achieving relevant biological time scales. As an alternative, multiscale models, which combine AA descriptions at particular regions with coarse-grained (CG) representations of the remaining components, were used to address this computational challenge while preserving the relevant details of the system. In this work, we implement a new multiscale approach based on the SIRAH force field to gain deeper insights into the dynamics of the LacI-DNA system. Our methodology allows for a dual resolution treatment of the solute and solvent, explicitly representing the protein, DNA, and solvent environment without compromising the AA region. Starting from the P1 loop configuration in an undertwisted conformation, we were able to observe the transition to the more stable overtwisted state. Additionally, a detailed characterization of the conformational space sampled by the DNA loop was done. In agreement with experimental and theoretical evidence, we observed the transient formation of kinks at the loop, which were stabilized by the presence of counterions at the minor groove. We also show that the loops intrinsic flexibility can account for reported FRET measurements and bent conformations required to bind the CAP transcription factor.

Assessing the Accuracy of the SIRAH Force Field to Model DNA at Coarse Grain Level

We present a comparison between atomistic and coarse grain models for DNA developed in our group, which we introduce here with the name SIRAH. Molecular dynamics of DNA fragments performed using implicit and explicit solvation approaches show good agreement in structural and dynamical features with published state of the art atomistic simulations of double stranded DNA (using Amber and Charmm force fields). The study of the multi-microsecond timescale results in counterion condensation on DNA, in coincidence with high-resolution X-ray crystals. This result indicates that our model for solvation is able to correctly reproduce ionic strength effects, which are very difficult to capture by CG schemes.

Isoform-specific determinants in the HP1 binding to histone 3: insights from molecular simulations

Despite the significant improvements in anti HIV-1 treatment, AIDS remains a lifelong disease due to the impossibility to eradicate the viral reservoir established upon integration of the viral genome. Controlling the epigenetic block imposed by the host cell machinery to the viral transcription may represent a therapeutic alternative to purge the viral reservoir, offering a way to eradicate the infection. Heterochromatin protein 1 (HP1) has been reported to actively participate in the silencing of HIV-1 integrated genome by binding to histone 3 (H3) tail. This interaction is mediated by the Chromodomain of HP1. Nevertheless, the structural features that determine its binding to H3 tail upon post-transductional modifications, such as methylation and phosphorylation as well as isoform-specific effects have not yet been described. We have undertaken the systematic simulation of the Chromodomains of the isoforms beta and gamma of HP1 in complex with the H3 tail methylated at Lys9 in presence/absence of phosphorylation at Ser10. Our results pinpoint isoform-specific electrostatic interactions as important determinants for the stability of the complexes. Characterization of intermolecular contacts between HP1 variants and H3 furnishes new insights on isoform-specific recognition and the effect of phosphorylation.

MD Simulations of Virus-Like Particles with Supra CG solvation affordable to desktop computers

Viruses are tremendously efficient molecular devices that optimize the packing of genetic material using a minimalistic number of proteins to form a capsid or envelope that protects them from external threats, being also part of cell recognition, fusion, and budding machineries. Progress in experimental techniques has provided a large number of high-resolution structures of viruses and viruslike particles (VLP), while molecular dynamics simulations may furnish lively and complementary insights on the fundamental forces ruling viral assembly, stability, and dynamics. However, the large size and complexity of these macromolecular assemblies pose significant computational challenges. Alternatively, Coarse-Grained (CG) methods, which resign atomistic resolution privileging computational efficiency, can be used to characterize the dynamics of VLPs. Still, the massive amount of solvent present in empty capsids or envelopes suggests that hybrid schemes keeping a higher resolution on regions of interest (i.e., the viral proteins and their surroundings) and a progressively coarser description on the bulk may further improve efficiency. Here we introduce a mesoscale explicit water model to be used in double- or triple-scale simulations in combination with popular atomistic parameters and the CG water used by the SIRAH force field. Simulations performed on VLPs of different sizes, along with a comprehensive analysis of the PDB, indicate that most of the VLPs so far reported are amenable to be handled on a GPU-accelerated desktop computer using this simulation scheme.

Structure-based, in silico approaches for the development of novel cAMP FRET reporters.

A significant contribution to the research in cAMP signaling has been made by the development of genetically encoded FRET sensors that allow detection of local concentrations of second messengers in living cells. Nowadays, the availability of a number of 3D structures of cyclic nucleotide-binding domains (CNBD) undergoing conformational transitions upon cAMP binding, along with computational tools, can be exploited for the design of novel or improved sensors. In this chapter we will overview some coarse-grained geometrical considerations on fluorescent proteins, CNBD, and linker peptides to draw simple qualitative rules that may aid the design of novel sensors. Finally, we will illustrate how the application of these simple rules can be used to describe the mechanistic basis of cAMP sensors reported in the literature.

The SIRAH force field 2.0: Altius, Fortius, Citius

A new version of the coarse-grained (CG) SIRAH force field for proteins has been developed. Modifications to bonded and non-bonded interactions on the existing molecular topologies significantly ameliorate the structural description and flexibility of a non-redundant set of proteins. The SIRAH 2.0 force field has also been ported to the popular simulation package AMBER, which along with the former implementation in GROMACS expands significantly the potential range of users and performance of this CG force field on CPU/GPU codes. As a non-trivial example of application, we undertook the structural and dynamical analysis of the most abundant and conserved calcium-binding protein, namely, Calmodulin (CaM). CaM is constituted by two calcium-binding motifs called EF-hands, which in presence of Calcium specifically recognize a cognate peptide by embracing it. CG simulations of CaM bound to four Calcium ions in the presence or absence of a binding peptide (holo and apo forms, respectively), resulted in good and stable ion coordination. The simulation of the holo form starting from an experimental structure sampled near-native conformations, retrieving quasi-atomistic precision. Removing the binding peptide enabled the EF-hands to perform large reciprocal movements, comparable to those observed in NMR structures. On the other hand, the isolated peptide starting from the helical conformation experienced spontaneous unfolding, in agreement with previous experimental data. However, repositioning the peptide in the neighborhood of one EF-hand not only prevented the peptide unfolding but also drove CaM to a fully bound conformation with both EF-hands embracing the cognate peptide, resembling the experimental holo structure. Therefore, SIRAH 2.0 showed the capacity to handle a number of structurally and dynamically challenging situations including metal ion coordination, unbiased conformational sampling, and specific protein-peptide recognition. TOC.