UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

Browse

Filters

2004 - 200912010 - 20152

Settings

Subject: search.filters.subject.channel

Search Results

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    DISCOVERY OF AN ESCHERICHIA COLI CHANNEL WITH HIGH VOLTAGE DEPENDENCE AND COOPERATIVITY.
    (2015) Lin, Shang-Hsuan; Colombini, Marco; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Channels are essential for controlling the permeability of cellular membranes. The regulation of channel gating therefore plays an important role physiologically. Voltage-gating is one of the regulations that cells utilize wherein the change in transmembrane potential cause conformational changes in channels. Here a novel bacterial channel from Escherichia coli with remarkable voltage-gating properties is reported. When the channel-forming protein was reconstituted into a planar phospholipid membrane, two different types of channel activities were observed. Type A is weakly cation-selective, with a single channel conductance about 1.5 nS (in 1M KCl solution), corresponding to a pore size of 0.9 nm. High positive voltages cause step-wise closures. Type B is voltage-independent, with much larger and noisier conductance. When LaCl3 was added, Type B channels first showed a decrease in conductance, and the residual conductance became voltage-gated, indistinguishable from Type A channels. Under triangular voltage waves, more interesting voltage-gating behaviors were revealed. The single conducting unit seems to be composed of three channels, each with the identical 1.5 nS conductance (namely channel (1), channel (2), and channel (3)). Based on the voltages at which they close/reopen, and the sequence of their closure/reopening, a model was proposed as follows. All three channels are proposed to be molecularly identical but, channel (1) and channel (3) have the same orientation, which is opposite to that of channel (2). Altogether these three channels form the conducting unit in a linear array. The voltage sensor domain of each channel is proposed to take the form of a dipole moment. The interaction between dipole moments of the channels, each with an opposite orientation with its neighbor(s), leads to the impressively high cooperativity between channels. Although the physiological roles of these channels are not clear yet, the remarkably steep voltage dependence (n~14) rivals that of the channels in excitable membranes.
  • Thumbnail Image
    Item
    Protein permeability pathways in the mitochondrial outer membrane during apoptosis
    (2012) Vidyaramanan, Ganesan; Colombini, Marco; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Apoptosis, a form of programmed cell death is a physiological, homeostatic process that guides the systematic removal of unwanted, dying or damaged cells from the body. A key step in apoptosis is the irreversible release of mitochondrial intermembrane space (IMS) proteins into the cytosol by a process called Mitochondrial Outer Membrane Permeabilization (MOMP). MOMP is regulated by a special class of proteins called Bcl-2 family proteins and a sphingolipid called ceramide. The pro-apoptotic Bcl-2 proteins, especially Bax and Bak can cooperate with ceramide to form channels in mitochondria that cause protein efflux during MOMP. The ability of ceramide to form protein-permeable channels in MOM is established. Bax and ceramide enhanced MOMP synergistically. The ability of Bax to stimulate ceramide channels was investigated. It was found that the apparent affinity of Bax for a ceramide channel increases with the ceramide channel size. The results indicate that Bax binds a small ceramide channel and drives its growth until the Bax molecule finds the best fit to the channel. This interaction between Bax and a ceramide channel does not require of the presence of other Bcl-2 proteins or mitochondrion-specific factors. The critical structural features of ceramide were investigated for their role in ceramide channel formation. Analogs of ceramide bearing modifications in the functional groups were analyzed for their ability to form channels to assess stability and also to interact with native ceramide to form channels to assess compatibility between interacting groups. The C1-hydroxyl was found to be indispensable for channel formation while the C3-hydroxyl was inconsequential to channel formation. The amide nitrogen with its ability to donate hydrogen was important for stability as methylating the nitrogen diminished the channel forming ability. Similarly, converting the carbonyl oxygen to a urea group, now more polar and a stronger hydrogen bond former resulted in more stable permeabilization. Changes to the hydrocarbon tails did not affect the ability to form channels. Phytoceramide, which has a C4 hydroxyl instead of the C4-C5 trans double bond formed stable channels but phytoceramide inhibited channel formation by ceramide suggesting incompatibility in structure. Bax activation involves translocation of Bax from the cytosol to the MOM, conformational changes and subsequent channel formation. All steps involved in Bax activation are not well-understood. We have used ionic strength as a modulating tool to dissect the different steps in Bax mediated MOMP. Increasing the ionic strength was found to delay formation of real-time permeability by Bax. Increasing the ionic strength resulted in smaller channels that grew in size slowly. The high permeability induced by low ionic strength was not reversed by raising the ionic strength suggesting that Bax channels are not in dynamic equilibrium with Bax monomers. Ionic strength also altered the sensitivity of Bax mediated MOMP to inhibition by Bcl-xL. Ionic strength, however did not affect Bax insertion into membranes. Thus, ionic strength presents a good diagnostic tool to modulate Bax mediated channel formation downstream of Bax insertion into membranes.
  • Thumbnail Image
    Item
    Characterization of the spatial differences in hydrological functioning in a tidal marsh, Patuxent River, MD: A framework for understanding nutrient dynamics
    (2004-12-08) Phemister, Karen; Prestegaard, Karen L.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This study investigates spatial variations in sediment hydraulic conductivity (K), network channel shape and horizontal groundwater flux magnitude toward tidal network channels in a freshwater tidal marsh. Results showed the average value of K at zero meters from the creekbank was significantly higher than the K at both 5 and 15 meters from the network channel creekbank. Creekbank gradient did increase with increasing distance from the main channel and some data indicated that channel width-to-depth ratio (F), which is inversely related to creekbank gradient, correlates well with K. In addition, horizontal groundwater flux magnitude at a depth of 11 cm was significantly greater than flux magnitude at 22 cm below the ground surface at the first-order network channel location. Horizontal flux magnitude was also significantly higher from 5 to 0 meters than from 15 to 5 meters from the network channel creekbank at both the first- and second-order channel locations.