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Daniel J. Müller
Characterizing molecular interactions driving the function of cells and cellular machineries
Previous and current research
What drives cellular machines and how can we apply our knowledge?
Molecular interactions drive all processes in life. They determine the molecular crosstalk and build the basic language of biological processes. In water-soluble and membrane proteins molecular interactions fold the polypeptide into the functional protein, stabilize the structure, or lead to protein misfolding. These molecular forces determine protein-protein interactions, switching on and off ion channels, ligand-binding, the functional states of receptors, and the supramolecular assembly of molecular machines to functional units. Because of this enormous importance it is one pertinent demand in life sciences to characterize how these interactions drive biological processes and thus to decipher fundamentals of the biological language. To do so, we have pioneered two bionanotechnological methods, single-molecule atomic force microscopy (AFM) and single-molecule force spectroscopy (SMFS), which allows detecting inter- and intramolecular interactions of native membrane proteins. Recent extensions of both methods allow to image cells at nanometer resolution and to study interactions of single cells at molecular resolution using single-cell force spectroscopy (SCFS).
Cartoon showing the detection of molecular interactions of single living cells. The bionanotechnological technique CellHesionTM developed in the laboratory of cellular machines is now internationally sold as the standard lab equipment to characterize cellular adhesion down to the contribution of single molecules.
Learning to detect and interpret molecular forces and interactions
SMFS and SCFS allow detecting inter- and intramolecular forces directly correlated to the strength of biomolecular interactions. Both methods reveal intriguing insights into the strength of forces that stabilize biological structures, determine various biological interactions, and drive biological processes. Modifications in experimental setups allow precisely locating interactions occurring within the macromolecule. In most cases this feasibility could be demonstrated on membrane proteins by determining the stability of secondary structures of (non-)functional proteins. It could be investigated how their stabilizing interactions alter upon environmental changes such as caused by temperature, electrolyte or protein-protein interactions. Molecular interactions occurring upon ligand-binding thereby activating a transporter could be detected and located in the protein structure.
Originating from the experimental setup of the force spectroscopy it was thought that the interactions detected would be mostly of mechanical nature. However, it has become clear that force spectroscopy detects molecular interactions resulting from different physical and chemical origins. The ongoing challenge is to dissect and determine the extent to which various physical interactions contribute to a certain force detected in biological systems. For example, what are the life-times of these interactions? How do they contribute to ligand- and inhibitor-binding? How do these interactions follow each other thereby forming an interaction network? How does deleting or adding an interaction (e.g. by mutations) affect complex interaction networks of membrane proteins? Typically such questions are experimentally addressed using techniques from structural biology, where structural details help decipher the nature of theses interactions. Since these techniques only provide static pictures they are inherently limited in addressing these questions. Certainly, with the existing and the forthcoming generations of SMFS and SCFS it will be possible to characterize and understand the forest of interactions and their contributions in switching the functional state of cell membranes and membrane proteins.
Future prospects and goals
We are at the frontier of understanding how biological systems are determined by their interactions. Many fundamental questions remain. For example:
- How do cells control adhesion and mechanics?
- How are the lifetimes of cellular interactions modulated?
- How do cellular interactions modulate the functional state of receptors?
- How do cells guide the assembly of proteins into functional units?
- Which and how do factors (e.g., chemical or genetic modifications, compounds) affect complex protein assemblies?
My scientific group has demonstrated that the current nanotechnological tools begin to answer some of these questions. It is our goal to further develop and apply nanotechnological tools to answer these and other pertinent questions.
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Selected publications
D.J. Müller & Y. Dufrene, Nature Nanotechnology (2008) 3, 261-269: Atomic force microscopy as a multifunctional molecular toolbox in nanobiotechnology’
J. Helenius, C.P. Heisenberg, H.E. Gaub & D.J. Müller, Journal of Cell Science (2008) 121, 1785-1791: Single-cell force spectroscopy
M. Krieg, Y. Arboleda, P.-H. Puech, J. Kaefer, F. Graener, D.J.
Müller & C.P. Heisenberg, Nature Cell Biology (2008) 10, 429-436: Quantifying adhesive and tensile cell properties determining germ layer organization during gastrulation
A. Engel & D.J. Müller, Nature Structural Biology (2000) 7, 715-718: Observing proteins at work with the atomic force microscope
F. Oesterhelt, D. Oesterhelt, M. Pfeiffer, A. Engel, H. Gaub & D.J. Müller, Science (2000) 288, 143-146:Unfolding pathways of individual bacteriorhodopsins
H. Seelert, A. Poetsch, N. Dencher, A. Engel, H. Stahlberg & D.J. Müller, Nature (2000) 405, 418-419: Proton powered turbine of a plant motor