Summary of research achievements

Structures of membrane proteins or membrane-related complexes that have been determined by members of our department include

• the plant light-harvesting complex, LHC-II at 3.4 Å resolution by electron crystallography (Kühlbrandt et al, Nature 1994), and 2.5 Å resolution by X-ray crystallography (Standfuss et al., EMBO J. 2005)
• a plant photosystem II subcomplex at 8 Å by electron crystallography (Rhee et al., Nature 1998)
• the plasma membrane proton ATPase from Neurospora crassa at 8 Å (Auer et al., Nature 1998)
• the bacterial sodium/proton antiporter NhaA as a dimer in the membrane, at 6 Å by electron crystallography (Williams, Nature 2000)
• the N intermediate of bacteriorhodopsin at 3.4 Å by electron crystallography (Vonck, EMBO J. 2000)
• the c subunit ring of the sodium-driven bacterial ATP synthase form Ilyobacter at 6 Å by electron crystallography (Vonck et al., J. Mol. Biol. 2002)
• the SecYEG bacterial protein translocase at 8 Å by electron crystallography (Breyton et al., Nature 2002)
• the c-subunit ring of Ilyobacter at 2.4 Å by X-ray crystallography (Meier et al, Science 2005)
• the outer membrane porin OmpG at 2.3 and 2.7 Å in the open and closed conformation, by X-ray crystallography (Yildiz et al, EMBO J 2006)
• the mitochondrial protein translocase TOM at 18 Å by single-particle cryo-EM (Model et al., J. Mol. Biol 2008)
• the osmoregulated osmolyte transporter BetP in the occluded conformation at 3.4 Å by X-ray crystallography (Ressl et al., Nature 2009)

In addition we have determined the high-resolution structures of several membrane-related proteins by X-ray crystallography, including

• the regulatory protein GlnK1 in three different functional states at 1.2 Å, 2.1 Å and 1.6 Å (Yildiz et al, EMBO J 2007)
• the ectoin-binding protein TeaA of the osmoregulated TRAP transport system of Halomonas elongata at 1.55 Å (Kuhlmann et al, Biochemistry 2008)
• the prokaryotic membrane-bound G-protein domain of the Feo iron transport system (Köster et al, J. Mol. Biol, in press)

By electron cryo-tomography we have shown that

• the mitochondrial ATP synthase forms dimer ribbons at the highly curved crista margins (Strauss et al, EMBO J. 2008)
• the chloroplast ATP synthase is, on the contrary, confined to minimally curved membranes of the thylakoid membrane (Daum et al, unpublished)

Biophysical studies

To complement our structural studies, we investigate the function of LHC-II, BetP, OmpG, and NhaP1 by a wide range of biophysical techniques, including uptake studies and electrophysiology, microspectroscopy (in collaboration with A. Royant , ESRF Grenoble, France), FTIR (in collaboration with W. Mäntele at Frankfurt University), and AFM (in collaboration with Daniel Müller, TU Dresden). The subunit stoichiometry of ATP synthase c-rings is characterized by LILBID mass spectrometry in collaboration with B. Brutschy (University of Frankfurt)

Current research

For a more detailed description of the research pursued in the Department, please refer to the web sites of the individual group leaders and staff scientists. Group leaders have obtained research grants to support their own projects, and supervise postdocs and students.

The main interests in Werner Kühlbrandt’s research group is electron and X-ray crystallography of membrane proteins in biological energy conversion and secondary transporters, and the native arrangement of large complexes in the membrane, studied by electron tomography.  The group has determined the structure of the plant light-harvesting complex LHC-II (Kühlbrandt et al, Nature 1994; Standfuss et al, EMBO J 2005). The structures of the dimeric sodium/proton antiporter NhaA (Williams et al, Nature 2000), and the bacterial protein translocase SecYEG (Breyton et al, Nature 2002) in the membrane have been determined at 6-8Å resolution by electron crystallography. These projects are being continued. Together with Janet Vonck, we work on the structure of large macromolecular assemblies, such as the fatty acid synthase, by single particle EM.

Two comparatively recent research themes in the group are electron cryo-tomography of biological membranes, and the development of a new phase contrast electron microscope for cryo-EM.  Electron tomography has revealed the arrangement of the ATP synthase in rows of dimes in the mitochondrial inner membrane, and the chloroplast ATP synthase in thylakoid membranes. In collaboration with H. Osiewacz at Frankfurt University we study the effects of aging on mitochondrial ultrastructure by cryo-tomography.  Bastian Barton and Mike Strauss are working with the new phase contrast electron microscope, and its applications to new biological questions.

The group of Thomas Meier is devoted to investigating the structure and function of the ATP synthase, in particular the F_o part in the membrane that drives rotary synthesis of ATP in the F₁ head using a proton or (in special cases) a Na⁺ gradient.  Thomas Meier has determined the first structure of a c-ring rotor (Meier et al, Science 2009) in the group of P. Dimroth at the ETH Zürich before he joined the department.  Since then, he has extended his studies to c-rings from a number of organisms, which have different subunit stoichiometries, and has characterized them by electron crystallography of 2D crystals, AFM in collaboration with the Müller group in Dresden, and by LILBID mass spectrometry in collaboration with Brutschy in Frankfurt. 

Eva Schäfer rejoined the department recently, after a 3 year postdoc abroad.  Her research interests include single-particle cryo-EM of large membrane protein complexes, especially the mitochondrial supercomplexes of the respiratory chain (Schäfer et al, Biochemistry 2007). 

Özkan Yildiz specializes in X-ray crystallography of membrane and membrane-associated proteins. He has determined the structure of the E. coli outer membrane porin OmpG in the open and closed conformation (Yildiz et al, EMBO J 2006), and the structure of the protein GlnK1 that regulates nitrogen uptake in bacteria and archaea in three different functional states (Yildiz et al, EMBO J 2007).  Most recently he and his group have solved the structure of the the GTP-binding domain of the ferrous iron uptake protein FeoB, the only known membrane-bound G-protein in prokaryotes (Köster et al, J. Mol. Biol.2009).

Christine Ziegler and her group focus on membrane transporters that maintain the osmotic pressure inside cells. BetP from the soil bacterium Corynebacterium glutamicum has the amazing ability of both sensing the internal osmotic pressure, and balancing it by specific uptake of glycine betaine, a small, inert molecule that acts as a compatible solute.  They determined the structure of the protein at 3.4 Å resolution by single-anomalous dispersion X-ray crystallography (Ressl et al, Nature 2009), which showed for the first a transporter in the occluded state. Surprisingly, the arrangement of membrane spanning helices was very similar to that found in other Na-dependent solute transporters, even though there was no sequence homology.  Other osmolyte transporters by Christine Ziegler include the TeaABC system, and stress-induced solute channels.