The elasticity of skin, lungs, major arteries, and other vertebrate tissues is imparted by the fibrous structural protein, elastin. c) We initially performed this study several years ago using a different force field combination (OPLS/AA with TIP3P) (Sarah Rauscher’s PhD thesis, 2012). 4) The comparison to experiment is rather weak and qualitative at best, and therefore should be strengthened. This result is expected, given the highly hydrophobic composition of the ELP sequence, and the fact that water is a poor solvent for hydrophobic residues. As such, the molecular basis for phase separation uncovered in this study is likely to be relevant to longer ELPs and full-length tropoelastin. All simulation parameters were identical to the other simulations. FTIR is a technique that is sensitive to changes in protein secondary structure and conformation [8–10]. The model of elastin-like aggregates derived here represents the first detailed model of a protein coacervate. However, the results presented in this manuscript are in excellent agreement with an NMR study by Reichheld et al. The question is that how can something unstable be a component of something so stable, like the triple helix structure of … b) Our choice of the CHARMM22* and CHARMM-refined TIP3P force fields is directly motivated by Rauscher et al. In each case, the dotted line indicates the power law fit to the data, with the exponent α provided next to each curve. The insoluble elastin matrix produced in the presence of ascorbate contains a somewhat higher content of hydroxyproline and lysine and slightly less lysine-derived cross-links than the non-ascorbate cultures. Long simulations of the aggregate were needed because of the slowed dynamics of interacting chains. Indeed, the present model system is not designed to capture all the properties of elastin. We have modified the text to address and clarify the force field issue as explained below: a) We cannot compare our results to experimental measurements performed on the same peptide, because to our knowledge such data are lacking for the (GVPGV)7 monomer. In order to delineate the equilibration phase of the simulation, we consider the running average of the radius of gyration and the total number of hydrogen bonds formed in the aggregate. Importantly, we find no crossover to a regime with α = 1/4, as one would expect if the chain were moving as if confined in a tube formed by the neighboring chains (that is, the entangled polymer melt regime described by de Gennes [de Gennes, 1979]). It is created by fibroblasts which are a type of connective tissues. Local interactions consist of sparse backbone hydrogen bonds and corresponding non-polar contacts. protein secondary structure at high pressures. 1) The simulation systems are highly simplified and may not adequately capture all relevant properties of real elastin. In addition to the replica exchange simulation, twenty 1 µs long simulations were carried out in the NPT ensemble at 298 K in order to carry out the analysis of chain dynamics and lifetimes of hydrogen-bonded turns. While the dynamics of turn formation are similar, non-local dynamics of the chain differs dramatically in solution and in the aggregate (Video 1). Shading in (a) and (b) indicates standard error. High conformational entropy is the key feature of the earliest model proposed for elastin’s structure: the random network model (Hoeve and Flory, 1958; Flory, 1974). To further examine the dependence of our results, particularly as regards the compactness of the isolated peptide in water, on the choice of force field, we repeated some of the simulations with a different force field combination. The concentration of the system was 102.9 mg/mL, which is higher than the concentration needed to observe aggregation of similar elastin peptides (Bellingham et al., 2001). KBP is a tetratricopeptide repeat-containing, right-handed α-solenoid that sequesters the kinesin motor domain’s tubulin-binding surface, structurally distorting the motor domain and sterically blocking its MT attachment. Shading indicates standard error. Elastin is a major protein component of tissues that require elasticity such as arteries, lungs, bladder, skin and elastic ligaments and cartilage. }, doi = {10.1021/acs.jpclett.5b01890}, journal = {Journal of Physical Chemistry Letters}, … We also provide snapshots of the final conformation of 5 of the MC simulations (Figure 3—figure supplement 1), as well as hydrogen bonding and non-polar contact maps of each of the 33 replicates (Figure 2—figure supplements 2 and 3), which demonstrate that each of the individual simulations also exhibits similar structural properties compared to the average results reported in the main text. The analysis of the average density of peptide and water from the center of mass of the aqueous monomer and of the peptide aggregate quantifies the presence of water throughout the system, even near the center of the peptide, where it represents over 0.2 g/cm3 in both systems (Figure 4). In all figures, error bars indicate standard error. 2) The significance of the results is not clear because important negative controls are missing. The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission. The high degree of hydration of the aggregate (40.0 ± 0.2% water content by mass, based on the number of water molecules hydrogen-bonded to the peptide) is consistent with experimental measurements of water content between 40% and 60% for elastin derived from various species (Chalmers et al., 1999). An expectedly high elastin content subsequent analysis,  ~39000 water molecules, and therefore should be performed in way... Self-Assembly does not entail formation of a well-ordered structure is unclear by Rauscher et al the chains associate form. 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