Quantitative all-atom and coarse-grained molecular dynamics simulation studies of DNA

The remarkable molecule that encodes genetic information for all life on earth—DNA—is a polymer with unusual physical properties. The mechanical and electrostatic properties of DNA are utilized extensively by cells in the replication, regular maintenance, and expression of their genetic material. This can be illustrated by considering the journey of a typical gene regulating protein, the lac repressor, which recognizes a particular gene and prevents its expression. First, the large electrostatic charge density of DNA provides an energetic track that guides the repressor’s search for its target binding site. Next, as the protein moves along the DNA, it attempts to deform the DNA. The repressor is only able to form an active complex with DNA that has the right sequence-dependent flexibility. Finally, the repressor is believed to form a very small DNA loop that prevents the gene from being expressed. The stability of the loop can be expected to depend sensitively on the global flexibility of DNA. Thus, the key to understanding the some of the most important cellular processes lies in understanding the physical properties of DNA. Single-molecule experiments allow direct observation of the behavior of individual DNA molecules, but act on length and timescales that are often too large and fast to observe underlying DNA and DNA–protein dynamics. Acting on length and timescales that complement single-molecule experiments, molecular dynamics simulations provide a high-resolution glimpse into the mechanics of a biomolecular world. Here, several simulation studies are presented, each of which quantified one or more properties of DNA. Specifically, the repulsive forces between parallel duplex DNA molecules were measured; the short-ranged, attractive end-to-end stacking energy was obtained; a single-stranded DNA model was developed that reproduced experimental measurements of its extension upon applied force; and finally the nature of single-stranded DNA binding to a single-stranded DNA binding protein was investigated. These works represent important steps towards larger simulations of more biologically complete DNA–protein systems

Structure of a Force-Conveying Cadherin Bond Essential for Inner-Ear Mechanotransduction

Hearing and balance use hair cells in the inner ear to transform mechanical stimuli into electrical signals. Mechanical force from sound waves or head movements is conveyed to hair-cell transduction channels by tip links, fine filaments formed by two atypical cadherins known as protocadherin 15 and cadherin 23. These two proteins are involved in inherited deafness and feature long extracellular domains that interact tip-to-tip in a $Ca^{2+}$-dependent manner. However, the molecular architecture of this complex is unknown. Here we combine crystallography, molecular dynamics simulations and binding experiments to characterize the protocadherin 15-cadherin 23 bond. We find a unique cadherin interaction mechanism, in which the two most amino-terminal cadherin repeats (extracellular cadherin repeats 1 and 2) of each protein interact to form an overlapped, antiparallel heterodimer. Simulations predict that this tip-link bond is mechanically strong enough to resist forces in hair cells. In addition, the complex is shown to become unstable in response to $Ca^{2+}$ removal owing to increased flexure of $Ca^{2+}$-free cadherin repeats. Finally, we use structures and biochemical measurements to study the molecular mechanisms by which deafness mutations disrupt tip-link function. Overall, our results shed light on the molecular mechanics of hair-cell sensory transduction and on new interaction mechanisms for cadherins, a large protein family implicated in tissue and organ morphogenesis, neural connectivity and cancer.Molecular and Cellular Biolog

Simultaneous use of solution, solid-state NMR and X-ray crystallography to study the conformational landscape of the Crh protein during oligomerization and crystallization

We explore, using the Crh protein dimer as a model, how information from solution NMR, solid-state NMR and X-ray crystallography can be combined using structural bioinformatics methods, in order to get insights into the transition from solution to crystal. Using solid-state NMR chemical shifts, we filtered intra-monomer NMR distance restraints in order to keep only the restraints valid in the solid state. These filtered restraints were added to solid-state NMR restraints recorded on the dimer state to sample the conformational landscape explored during the oligomerization process. The use of non-crystallographic symmetries then permitted the extraction of converged conformers subsets. Ensembles of NMR and crystallographic conformers calculated independently display similar variability in monomer orientation, which supports a funnel shape for the conformational space explored during the solution-crystal transition. Insights into alternative conformations possibly sampled during oligomerization were obtained by analyzing the relative orientation of the two monomers, according to the restraint precision. Molecular dynamics simulations of Crh confirmed the tendencies observed in NMR conformers, as a paradoxical increase of the distance between the two β1a strands, when the structure gets closer to the crystallographic structure, and the role of water bridges in this context

Structural basis of outer membrane protein insertion by the BAM complex

All Gram-negative bacteria, mitochondria and chloroplasts have outer membrane proteins (OMPs) that perform many fundamental biological processes. The OMPs in Gram-negative bacteria are inserted and folded into the outer membrane by the β-barrel assembly machinery (BAM). The mechanism involved is poorly understood, owing to the absence of a structure of the entire BAM complex. Here we report two crystal structures of the Escherichia coli BAM complex in two distinct states: an inward-open state and a lateral-open state. Our structures reveal that the five polypeptide transport-associated domains of BamA form a ring architecture with four associated lipoproteins, BamB–BamE, in the periplasm. Our structural, functional studies and molecular dynamics simulations indicate that these subunits rotate with respect to the integral membrane β-barrel of BamA to induce movement of the β-strands of the barrel and promote insertion of the nascent OMP

Insights into the structure and dynamics of lysyl oxidase propeptide, a flexible protein with numerous partners

Lysyl oxidase (LOX) catalyzes the oxidative deamination of lysine and hydroxylysine residues in collagens and elastin, which is the first step of the cross-linking of these extracellular matrix proteins. It is secreted as a proenzyme activated by bone morphogenetic protein-1, which releases the LOX catalytic domain and its bioactive N-terminal propeptide. We characterized the recombinant human propeptide by circular dichroism, dynamic light scattering, and small-angle X-ray scattering (SAXS), and showed that it is elongated, monomeric, disordered and flexible (Dmax: 11.7 nm, Rg: 3.7 nm). We generated 3D models of the propeptide by coarse-grained molecular dynamics simulations restrained by SAXS data, which were used for docking experiments. Furthermore, we have identified 17 new binding partners of the propeptide by label-free assays. They include four glycosaminoglycans (hyaluronan, chondroitin, dermatan and heparan sulfate), collagen I, cross-linking and proteolytic enzymes (lysyl oxidase-like 2, transglutaminase-2, matrix metalloproteinase-2), a proteoglycan (fibromodulin), one growth factor (Epidermal Growth Factor, EGF), and one membrane protein (tumor endothelial marker-8). This suggests new roles for the propeptide in EGF signaling pathway