Sidekick for membrane simulations: automated ensemble molecular dynamics simulations of transmembrane helices

The interactions of transmembrane (TM) α- helices with the phospholipid membrane and with one another are central to understanding the structure and stability of integral membrane proteins. These interactions may be analyzed via coarse grained molecular dynamics (CGMD) simulations. To obtain statistically meaningful analysis of TM helix interactions, large (N ca. 100) ensembles of CGMD simulations are needed. To facilitate the running and analysis of such ensembles of simulations, we have developed Sidekick, an automated pipeline software for performing high throughput CGMD simulations of α-helical peptides in lipid bilayer membranes. Through an end-to-end approach, which takes as input a helix sequence and outputs analytical metrics derived from CGMD simulations, we are able to predict the orientation and likelihood of insertion into a lipid bilayer of a given helix of a family of helix sequences. We illustrate this software via analyses of insertion into a membrane of short hydrophobic TM helices containing a single cationic arginine residue positioned at different positions along the length of the helix. From analyses of these ensembles of simulations, we estimate apparent energy barriers to insertion which are comparable to experimentally determined values. In a second application, we use CGMD simulations to examine the self-assembly of dimers of TM helices from the ErbB1 receptor tyrosine kinase and analyze the numbers of simulation repeats necessary to obtain convergence of simple descriptors of the mode of packing of the two helices within a dimer. Our approach offers a proof-of-principle platform for the further employment of automation in large ensemble CGMD simulations of membrane proteins

Piezo1 Forms Specific, Functionally Important Interactions with Phosphoinositides and Cholesterol

Touch, hearing, and blood pressure regulation require mechanically gated ion channels that convert mechanical stimuli into electrical currents. One such channel is Piezo1, which plays a key role in the transduction of mechanical stimuli in humans and is implicated in diseases, such as xerocytosis and lymphatic dysplasia. There is building evidence that suggests Piezo1 can be regulated by the membrane environment, with the activity of the channel determined by the local concentration of lipids, such as cholesterol and phosphoinositides. To better understand the interaction of Piezo1 with its environment, we conduct simulations of the protein in a complex mammalian bilayer containing more than 60 different lipid types together with electrophysiology and mutagenesis experiments. We find that the protein alters its local membrane composition, enriching specific lipids and forming essential binding sites for phosphoinositides and cholesterol that are functionally relevant and often related to Piezo1-mediated pathologies. We also identify a number of key structural connections between the propeller and pore domains located close to lipid-binding sites

Multiscale Simulations Suggest a Mechanism for the Association of the Dok7 PH Domain with PIP-Containing Membranes

Dok7 is a peripheral membrane protein that is associated with the MuSK receptor tyrosine kinase. Formation of the Dok7/MuSK/membrane complex is required for the activation of MuSK. This is a key step in the complex exchange of signals between neuron and muscle, which lead to neuromuscular junction formation, dysfunction of which is associated with congenital myasthenic syndromes. The Dok7 structure consists of a Pleckstrin Homology (PH) domain and a Phosphotyrosine Binding (PTB) domain. The mechanism of the Dok7 association with the membrane remains largely unknown. Using multi-scale molecular dynamics simulations we have explored the formation of the Dok7 PH/membrane complex. Our simulations indicate that the PH domain of Dok7 associates with membranes containing phosphatidylinositol phosphates (PIPs) via interactions of the β1/β2, β3/β4, and β5/β6 loops, which together form a positively charged surface on the PH domain and interact with the negatively charged headgroups of PIP molecules. The initial encounter of the Dok7 PH domain is followed by formation of additional interactions with the lipid bilayer, and especially with PIP molecules, which stabilizes the Dok7 PH/membrane complex. We have quantified the binding of the PH domain to the model bilayers by calculating a density landscape for protein/membrane interactions. Detailed analysis of the PH/PIP interactions reveal both a canonical and an atypical site to be occupied by the anionic lipid. PH domain binding leads to local clustering of PIP molecules in the bilayer. Association of the Dok7 PH domain with PIP lipids is therefore seen as a key step in localization of Dok7 to the membrane and formation of a complex with MuSK

Studies of the MuSK system using molecular dynamics simulations

Formation and maintenance of neuromuscular junctions, as well as signalling from the nerve to the muscle, depends on the concerted action of a number of membrane proteins. Muscle-Specific Kinase (MuSK) and Downstream-of-Kinase 7 (Dok7) are key players in these processes. MuSK acts as the central component in the complexes responsible for nervemuscle communication. Dok7 then regulates full activation of MuSK. Despite available structural data for these proteins, the structure of the overall Dok7/MuSK complex in a cell membrane remains elusive. In this thesis, a multi scale molecular simulation approach (combining coarsegrained and atomistic simulations) was used to study the formation of the Dok7/MuSK/membrane complex. Three major insights into this process were obtained. Firstly, dimerisation of the transmembrane helix of MuSK is mediated by a small-x-x-x-small amino acid motif, and it is not affected significantly by the addition of anionic lipids or unstructured extensions on each end of the helices. Secondly, Dok7 interacts with PIP-containing bilayers via a positive patch on its pleckstrin-homology (PH) domain. Thirdly, it is shown that MuSK's behaviour in a complex model membrane is potentially influenced by the presence of Dok7. Taken together, these results allow formulation of a defined model of the structure and mechanism of MuSK in a model membrane.</p

Voltage-Gated Sodium Channel Pharmacology: Insights From Molecular Dynamics Simulations

Voltage-gated ion channels are the target of a range of naturally occurring toxins and therapeutic drugs. There is a great interest in better understanding how these diverse compounds alter channel function in order to design the next generation of therapeutics that can selectively target one of the channel subtypes found in the body. Since the publication of a number of bacterial sodium channel structures, molecular dynamics simulations have been invaluable in gaining a high resolution understanding where many of these small molecules and toxins bind to the channels, how they find their binding site, and how they can selectively bind to one channel subtype over another. This chapter summarizes these recent studies to highlight what has been learnt about channel pharmacology using computer simulations and to draw out shared conclusions, focusing separately on toxin–channel interactions and small molecule–channel interaction

Protonation state of inhibitors determines interaction sites within voltage-gated sodium channels

Voltage-gated sodium channels are essential for carrying electrical signals throughout the body, and mutations in these proteins are responsible for a variety of disorders, including epilepsy and pain syndromes. As such, they are the target of a number of drugs used for reducing pain or combatting arrhythmias and seizures. However, these drugs affect all sodium channel subtypes found in the body. Designing compounds to target select sodium channel subtypes will provide a new therapeutic pathway and would maximize treatment efficacy while minimizing side effects. Here, we examine the binding preferences of nine compounds known to be sodium channel pore blockers in molecular dynamics simulations. We use the approach of replica exchange solute tempering (REST) to gain a more complete understanding of the inhibitors’ behavior inside the pore of NavMs, a bacterial sodium channel, and NavPas, a eukaryotic sodium channel. Using these simulations, we are able to show that both charged and neutral compounds partition into the bilayer, but neutral forms more readily cross it. We show that there are two possible binding sites for the compounds: (i) a site on helix 6, which has been previously determined by many experimental and computational studies, and (ii) an additional site, occupied by protonated compounds in which the positively charged part of the drug is attracted into the selectivity filter. Distinguishing distinct binding poses for neutral and charged compounds is essential for understanding the nature of pore block and will aid the design of subtype-selective sodium channel inhibitors

Average radial distribution function of PIP₂ and PIP₃ around Dok7’s PH domain.

<p>The average, over the 20 repeat simulations, radial distribution function of the coarse-grained simulations with PIP lipids. Averages for the PC/PS lipids were taken from the PC/PS/PIP₂ simulations, whilst the PIP₃ line is taken from the PC/PS/PIP₃ simulations. The PC line is red, PS in green, PIP₂ in purple, and PIP₃ in orange.</p

Summary of the simulations.

<p>Summary of the simulations.</p