Research Topics - Membrane Biology

Exchange of materials and information between a living cell and its environment is mediated and regulated by membrane proteins. These proteins are involved in the regulation of electrical activity of the cell, transport of water and water soluble materials across the membrane, and production of ATP. Membrane receptors are the sites for detection informational signals, such as neurotransmitters and hormones, light, and even mechanical stress. The atomic-resolution structures of a few membrane proteins have been solved, and recent advances in structure determination of membrane proteins promise more structure to be solved soon. Our group studies the mechanism of function of membrane proteins with various computational methodologies. The proteins are simulated in their natural environment, i.e., embedded in fully hydrated patches of lipid bilayers and under constant pressure and temperature conditions. The main objective is to understand how specific structural motifs and/or chemical interactions in a protein play a role in its function.

BAR domain

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Living cells are characterized by a great diversity of separate internal spaces, the boundaries of which are made of membranes forming convoluted surfaces of manifold shapes. Sculpting these shapes is achieved in many cases by proteins. A single protein is too small to bend the membrane into useful shapes, such as spheres or tubes, that measure 10-100 nm, or more, in diameter. Indeed, the proteins work in teams, but exactly how remained a mystery. Now a computational study elucidates the membrane-sculpting process for proteins called amphiphysin N-BAR domain. Simulations performed with NAMD had revealed a first glimpse earlier (see the Sep 2008 highlight). The new study showed that multiple N-BAR domains form lattices maintained through electrostatic interactions. Positively charged, banana-shaped surfaces of individual proteins bend the negatively charged membrane, while the lattice formation ensures a uniform bending force across a wide membrane surface. In a dramatic example of computational "microscopy" the 200 microsecond sculpting of a large flat membrane into a complete tube was observed. More here.

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Proteins are the workers in cells; they carry out designated cellular functions tirelessly throughout their lifetimes. Some proteins can even hold two different jobs. One example of a dual-duty protein is the bacterial photosynthetic core complex. The photosyntehtic core complex performs the first steps of photosynthesis: absorption of sunlight and processing of light energy. Besides providing solar power, the core complex acts as an architect of the cell by shaping membranes in the interior of photosynthetic bacteria. Combining computational modeling and electron microscopy data using the Molecular Dynamics Flexible Fitting method, computational biologists have recently reported studies of both functions of the core complex, namely, the light-absorbing features and the membrane-sculpting properties. More details can be found on our photosynthetic chromatophore website.

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Biological cells are surrounded by a highly versatile, yet very feeble cellular membrane and need to balance differences between the cell's interior and exterior that otherwise would burst the membrane. For example, the osmotic pressures inside and outside the cell need to be closely balanced. Thousands of proteins in the membrane act as gatekeepers, opening pores that can also act as safety valves, helping to reduce the interior-exterior difference in pressure rapidly. One such protein, the mechanosensitive channel of small conductance MscS (see the Mar 2008 highlight, "Observation and Simulation Depict Cell's Safety Valve", the Feb 2007 highlight, "Observing and Modeling a Crucial Membrane Channel", the May 2006 highlight, "Electrical Safety Valve", and the Nov 2004 highlight, "Japanese Lantern Protein") opens in response to cellular membrane tension generated due to a drastic imbalance in osmotic pressure as it arises when a bacterial cell suddenly finds itself in fresh water, rather than a highly saline physiological medium. The MscS channel widens then to jettison molecules out of the cell and reduce tension on the cellular membrane quickly. In a recent report, researchers have combined experimental data from electron paramagnetic measurements and computer modeling to reveal in atomic detail how MscS opens and closes its channel. Combining measurement and modeling, the researchers established a highly resolving computational microscope, unmatched by existing microscopes (more on our MscS website).
Water Bipolar Arrangement in Aquaporins

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Aquaporins are membrane water channels that play critical roles in controlling the water contents of cells. These channels are widely distributed in all kingdoms of life, including bacteria, plants, and mammals. More than ten different aquaporins have been found in human body, and several diseases, such as congenital cataracts and nephrogenic diabetes insipidus, are connected to the impaired function of these channels. They form tetramers in the cell membrane, and facilitate the transport of water and, in some cases, other small solutes across the membrane. However, the water pores are completely impermeable to charged species, such as protons, a remarkable property that is critical for the conservation of membrane's electrochemical potential, but paradoxical at the same time, since protons can usually be transfered readily through water molecules. The results of our simulations have now provided new insight into the mechanism underlying this fascinating property. Water molecules passing the channel are forced, by the protein's electrostatic forces, to flip at the center of the channel (see the animation), thereby breaking the alternative donor-acceptor arrangement that is necessary for proton translocation (read the complete story in our Science paper).

Spherical HDL

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Cholesterol maintains a healthy body, but too much cholesterol can lead to atherosclerosis and heart disease. Lipoproteins can bag superfluous cholesterol in the arteries and transport it to the liver for removal. One such lipoprotein is high-density lipoprotein (HDL) which self-assembles into discoidal particles (see the Mar 2007 highlight) and then bags cholesterol. How this works is the subject of a recent report. Molecular dynamics simulations using NAMD revealed that discoidal HDL particles, teaming up with the enzyme LCAT, first turn cholesterol chemically into cholesterol ester and then sucks it into the interior of the particle; in the course of this process, the HDL particle swells into a sphere. More information on our lipoprotein website.
Ion permeation in K channels

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movie 1 ( 1.6MB ),
movie 2 ( 4.7MB ), movie 3 ( 1.4MB )

Biological cells, in particular neurons, maintain an inside-outside voltage gradient through active transport of ions (Na+, K+, Cl-, and others) across their membranes. The flow of the ions down their gradients through membrane channels is highly selective for each ion. The high selectivity permits nerve cells to signal each other through voltage spikes, which are produced through transient changes of channel conductivities for Na+ ions (channels open and close in about a ms) and K+ ions (channels open and close in about 10 ms). Crucial for the generation of voltage spikes is the selective, yet quick, conduction of ions, but as one knows from personal experience at border crossings, high selectivity and quick crossing seem to be mutually exclusive. Yet biological ion channels reconcile selectivity and speed. Prior experimental work, primarily that of year 2003 Nobelist MacKinnon, as well as computational work suggested how potassium channels achieve selectivity and speed. But until recently no high resolution atomic structure of a potassium channel was known in the open form and the suggested mechanism could not be tested under natural conditions through atomic level simulations. Last year's solution of the structure of the potassium channel Kv1.2 in its open form made it finally possible to simulate, using NAMD, the conduction of ions through Kv1.2 driven by a voltage gradient. The results reported recently confirmed indeed the high selectivity - high speed mechanism suggested earlier, namely a billiard-type motion of two and three ions, the last ion kicking the first ion out. The simulations revealed for the first time, through movies, the overall permeation process, including the jumps of ions between energetically favorable binding sites and the sequence of multi-ion configurations involved in permeation. More on our potassium channel web site.
Opening of Lacy'S cytoplasmic cavity

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Escherichia coli are bacteria living in the intestines of mammals as part of their healthy gut flora, but also causing disease outside of the gut. The bacteria import from their environment nutriments, for example molecules of lactose, a sugar. For this purpose Escherichia coli employs in its cell membrane a protein channel, lactose permease, that translocates the sugar outside-in. This is the bacterium's "sweet tooth". To establish the unidirectional sugar transport, the bacterium utilizes an electrical potential maintained in the form of a trans-membrane proton gradient (more protons on the outer cellular than on the inner cellular side of the membrane). Protons, very small ions, that enter the channel from the outside one at a time, open the outer channel entrance. This permits access of lactose that gets bound inside the channel. Release of the proton to the cell interior closes the outer channel entrance and opens the inner channel entrance, such that the bound lactose can enter the cell. Despite extensive and elegant biochemical studies, the physical mechanism that couples unidirectional proton and sugar translocation is not yet known in detail. A crystallographic structure of lactose permease permitted now investigations into this mechanism by means of molecular dynamics simulations using NAMD. The simulations, reported in a recent publication, showed one step of the proton - sugar translocation, namely how binding and unbinding of the proton activates a spring-like bond, a so-called salt bridge, that closes and opens the inner channel exit. More information on the lactose permease project can be found here.

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Living cells contain millions of proteins composed of sequences of different amino acids that typically fold spontaneously into well-defined three-dimensional conformations and then carry out their role as molecular machines serving manifold functions in the cells (see the protein folding highlight). The synthesis of the proteins is carried out by the ribosome, one of the largest molecular machines present in all cells, which reads the cell's genetic information for the purpose. Three researchers were recently awarded the 2009 Nobel Prize in Chemistry for the determination of the ribosome's structure. The physical mechanism of the ribosome, the cell's protein factory, is still largely unknown. Just as in any factory, there are multiple directions and controls on the protein assembly line. Sometimes the protein products need to be redirected to different parts of the cell, and other times assembly needs to be halted altogether. Now, significant new insights into both of these aspects of protein assembly have been made by combining electron microscopy with molecular dynamics simulations using the recently developed molecular dynamics flexible fitting method (MDFF, see the June 2008 highlight). This combination allowed researchers to visualize the complex between the ribosome and a protein-conducting channel that directs proteins into and across membranes for both a mammalian system (reported here) and a bacterial system (reported here). Amazingly, despite the evolutionary distance between mammals and bacteria, both complexes are remarkably similar. Simulations of the bacterial ribosome-channel complex, among the largest ever performed, further revealed the steps in the direction process. In a third study (reported here), researchers determined how TnaC, as a protein newly synthesized by the ribosome, can stall the ribosome from within during its own assembly, which then controls the expression of related genes. More information on these unique protein assembly controls can be found on our ribosome and our protein-conducting channel websites.
Electrostatic potential map of alpha-hemolysin

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In a biological cell, membrane channels act like miniature valves regulating the flow of ions and other solutes between intracellular compartments and across the cell's boundary. Assembled in complex circuits, they generate, transmit, and amplify signals orchestrating cell function. To investigate how membrane channels work, life scientists, using an extremely fine pipette, isolate a tiny patch of a cell membrane and, in so-called patch clamp measurements, determine electric currents in response to applied electric potentials. Dramatic increase in computational power and its efficient utilization by NAMD allows one today to reproduce such studies computationally, calculating the permeability of a membrane channel to ions and water directly from its atomic structure. In what is one of the largest molecular dynamics simulation to date, described in a recent paper as well as on our web site (here), one copy of the membrane channel alpha-hemolysin, submerged in a lipid membrane and water, was subject to an external electric field that drove ions and water through the channel. The calculations produced also an image of the electrostatic potential across the channel (see figure).


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Bacteria contain the simplest photosynthetic machineries found in nature. Higher organisms like algae and plants practice photosynthesis in a more elaborate but principally similar manner as bacteria. But even for its simplicity, the bacterial photosynthetic unit is not without its unsolved mysteries. Take, for example, the crucial photosynthetic core complex, which performs light absorption and the initial processing of the light energy. In certain bacterial species, the core complex contains two copies of an additional small protein (made of about 80 amino acids) called PufX, whose role in photosynthesis is still a puzzle, and its location within the core complex is yet to be pinpointed. Numerous imaging studies have been published, yielding two opinions on what the role of PufX is and where exactly it resides. One opinion assigns the protein the role of gate keeper, the other the role of coordinator. Recently, a computational investigation was carried out that much supports the second role. Since PufX comes as a pair, two copies of PufX were placed side-by-side in a biological membrane and they were seen to adhere to each other strongly, but assume with their cylindrical (helical) shape an angle of 38 degrees. This geometry is perfectly suited for PufX to join the two parts of the symmetrical core complex together in the middle and to impose on the parts the tilt that was actually observed in the imaging studies. The needed simulations were done with NAMD. More details can be found on our photosynthetic core complex website.
BAR domain

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Olga Svinarski and VMD

The cells of higher life forms, so-called eukaryotic cells, are subdivided through many internal membranes made of lipid bilayers. The internal membranes assume numerous shapes, like spheres, tubes or parallel sheets. Outside of cells, biological membranes adopt usually flat shapes and the question arises, how do eukaryotic cells sculpt their inner membranes? The question of flat membrane sculpting is particularly interesting also as mature cells constantly produce new membrane shapes, for example spherical vesicles filled with certain biomolecules destined for release into the extracellular space, a process called exocytosis. The cell has many mechanisms available for sculpting its membranes, one of them relying on proteins called BAR domains that act from the surface of lipid bilayers. Molecular modeling with NAMD and VMD has provided valuable views of BAR domains at work in case of the so-called N-BAR family (see the earlier highlights Protein Teamwork, Jun 2009 and Proteins Sculpting Cell Interior, Sep 2008). Researchers report now an extension of the earlier studies to the F-BAR domain family of membrane sculpting proteins. The new modeling work is particularly exciting as it can be directly compared to electron microscopy images of membrane tubes sculpted from flat membranes in experiments done outside of cells. The new studies reveal how F-BAR domains sculpt tubular membranes through the shape of dimerized domains and through F-BAR domains not acting individually, but as an army of F-BAR domains adopting an ordered formation on one side of the membrane. More on our F-BAR domain web page.

p7 viroporin

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All living systems contain proteins whose job is to move ions across a lipid membrane. Even viruses encode ion transport proteins, which they need to complete their lifecycle and release themselves from infected cells. Such proteins, called viroporins, usually consist of small subunits of one or two helices that can self-assemble in a lipid bilayer into a pore-like structure. Although in some cases, the resulting structures resemble the well-ordered, selective ion channels in higher organisms, often they take on a more disordered character, forming pores with variable numbers of subunits, which adapt their structure and behavior to the environment in which they find themselves. This inherent flexibility and disorder makes it very difficult to produce high-resolution crystal structures of viroporins, which is unfortunate, since they could offer attractive drug targets for new antiviral therapies. Computational modelling and molecular dynamics simulations can help fill in the gaps in our structural knowledge of viroporins, and provide plausible 3-D models for visualization and drug design. In a recent publication, scientists published models of the p7 viroporin found in Hepatitis C virus. MD simulations of these models revealed that p7 can form stable pores with 4 to 7 subunits, with a bias towards 6 or 7 subunits, and that the p7 oligomers are highly flexible in adapting to different membrane thicknesses. These simulations also suggested that specific amino acids in certain places in the structure could play a role in controlling the ion permeability of p7. More details can be found on our p7 website.

light harvesting

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Photosynthesis, the main source of energy for all life, is performed by an intricate assembly of hundreds of proteins, which harvest, transfer, convert, and store solar energy. The simplest such light harvesting machine is the purple bacterial photosynthetic unit (PSU), which performs anoxygenic photosynthesis and is significantly simpler and evolutionarily more primitive than its counterpart found in plants. The bacterial PSU is organized in the form of a pseudo-spherical membrane domain of approximately 60 nm diameter called a chromatophore vesicle.