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Investigation of Biological Macromolecules Using Atomic Force Microscope‐Based Techniques

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Investigation of Biological Macromolecules Using Atomic Force Microscope‐Based Techniques Investigation of Biological Macromolecules Using Atomic Force Microscope‐Based Techniques Dissertation zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr. rer. nat.) vorgelegt der Fakultät Mathematik und Naturwissenschaften an der Technischen Universität Dresden von Christian A. Bippes geboren am 19.11.1978 in Karlsruhe Datum der Einreichung: 18.05.2009 1. Gutachter: Prof. Dr. Daniel J. Müller 2. Gutachter: Prof. Dr. Dimitrios Fotiadis Datum der Disputation: 18.08.2009 Meinen Eltern. SUMMARY The atomic force microscope (AFM) provides a powerful instrument for investigating and manipulating biological samples down to the subnanometer scale. In contrast to other microscopy methods, AFM does not require labeling, staining, nor fixation of samples and allows the specimen to be fully hydrated in buffer solution during the experiments. Moreover, AFM clearly compares in resolution to other techniques. In general, the AFM can be operated in an imaging or a force spectroscopy mode. In the present work, advantage was taken of this versatility to investigate single biomolecules and biomolecular assemblies. A novel approach to investigate the visco-elastic behavior of biomolecules under force was established, using dextran as an example. While a molecule tethered between a solid support and the cantilever tip was stretched at a constant velocity, the thermally driven oscillation of the cantilever was recorded. Analysis of the cantilever Brownian noise provided information about the visco-elastic properties of dextran that corresponded well to parameters obtained by alternative methods. However, the approach presented here was easier to implement and less time-consuming than previously used methods. A computer controlled force-clamp system was set up, circumventing the need for custom built analogue electronics. A commercial PicoForce AFM was extended by two computers which hosted data acquisition hardware. While the first computer recorded data, the second computer drove the AFM bypassing the manufacturer's microscope control software. To do so, a software-based proportional-integral- differential (PID) controller was implemented on the second computer. It allowed the force applied to a molecule to be held constant over time. After tuning of the PID controller, response times obtained using that force-clamp setup were comparable to those of the recently reported analogue systems. The performance of the setup was demonstrated by force-clamp unfolding of a pentameric Ig25 construct and the membrane protein NhaA. In the latter case, short-lived unfolding intermediates that were populated for less than 10 ms, could be revealed. Conventional single-molecule dynamic force spectroscopy was used to unfold the serine:threonine antiporter SteT from Bacillus subtilis, an integral membrane protein. Unfolding force patterns revealed the unfolding barriers stabilizing structural segments of SteT. Ligand binding did not induce new unfolding barriers suggesting that weak interactions with multiple structural segments were involved. In contrast, ligand binding caused changes in the energy landscape of all structural segments, thus turning the protein from a brittle, rigid into a more stable, structurally flexible conformation. Functionally, rigidity in the ligand-free state was thought to facilitate specific ligand binding, while flexibility and increased stability were required for conformational changes associated with substrate translocation. These results support the working model for transmembrane transport proteins that provide alternate access of the binding site to either face of the membrane. Finally, high-resolution imaging was exploited to visualize the extracellular surface of Cx26 gap junction hemichannels (connexons). AFM topographs reveal pH- dependent structural changes of the extracellular connexon surface in presence of HEPES, an aminosulfonate compound. At low pH (< 6.5), connexons showed a narrow and shallow channel entrance, which represented the closed pore. Increasing pH values resulted in a gradual opening of the pore, which was reflected by increasing channel entrance widths and depths. At pH > 7.6 the pore was fully opened and the pore diameter and depth did not increase further. Importantly, coinciding with pore gating a slight rotation of the subunits was observed. In the absence of aminosulfonate compounds, such as HEPES, acidification did not affect pore diameters and depths, retaining the open state. Thus, the intracellular concentration of taurine, a naturally abundant aminosulfonate compound, might be used to tune gap junction sensitivity at low pH. Table of Contents TABLE OF CONTENTS CHAPTER 1 INTEGRAL MEMBRANE PROTEINS..................................................................................1 1.1 CELLULAR MEMBRANES........................................................................................... 1 1.2 MEMBRANE PROTEINS ............................................................................................. 3 1.2.1 On the Importance of Membrane Protein Folding and Assembly.......................5 1.2.1.1 Progress in Protein Folding ...................................................................................5 1.2.1.2 Membrane Protein Folding ....................................................................................7 Translocon‐Assisted Membrane Protein Folding and Insertion.........................7 The ʺTwo‐Stageʺ Model .....................................................................................8 The ʺFour‐Stepʺ Model ......................................................................................9 1.2.1.3 Membrane Protein Misfolding .............................................................................11 1.2.2 Hurdles in Membrane Protein Studies ...................................................................12 1.2.3 Methodological Approaches in Membrane Protein Research ............................13 1.3 MEMBRANE PROTEINS FULLFIL SPECIFIC FUNCTIONS....................................... 15 1.3.1 Gap Junctions...............................................................................................................15 1.3.2 Amino Acid Transporters ..........................................................................................18 1.3.2.1 How Cells Perform Transport Across Membranes ..............................................18 1.3.2.2 L‐Amino Acid Transport......................................................................................19 CHAPTER 2 ATOMIC FORCE MICROSCOPY......................................................................................21 2.1 HISTORY................................................................................................................... 21 2.2 PRINCIPLE ................................................................................................................ 21 2.2.1 AFM Setup....................................................................................................................21 2.2.2 Cantilevers....................................................................................................................23 2.2.2.1 General Considerations ........................................................................................23 2.2.2.2 Forces Acting on the Cantilever...........................................................................24 2.2.2.3 Force Sensitivity...................................................................................................26 2.2.2.4 Cantilever Calibration..........................................................................................27 Detector Calibration..........................................................................................27 Spring Constant Calibration – Added Mass.....................................................28 Spring Constant Calibration – Sader Method ..................................................29 Spring Constant Calibration – Thermal Fluctuation Analysis ........................29 2.3 OPERATION MODES ............................................................................................... 30 2.3.1 Surface Imaging...........................................................................................................31 2.3.1.1 Contact Mode .......................................................................................................31 2.3.1.2 Tapping Mode ......................................................................................................32 2.3.2 Force Measurement.....................................................................................................33 2.3.2.1 AFM‐Based Single‐Molecule Force Spectroscopy................................................35 i Table of Contents Constant Velocity Force Spectroscopy.............................................................. 35 Constant Force Force Spectroscopy.................................................................. 36 2.3.2.2 Polymer Extension Models .................................................................................. 36 Freely Jointed Chain Model.............................................................................. 37 Wormlike Chain Model .................................................................................... 37 2.3.2.3 Model Systems..................................................................................................... 38 Dextran............................................................................................................
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