Our group investigates the structure and organization of membrane proteins by electron cryo-microscopy and crystallography to find out how they function in the cell. Most of the proteins or membranes we examine are prepared within the group by molecular biology and biochemical techniques. Of central interest are the membrane proteins and membrane complexes active in biological energy conversion in mitochondria, chloroplasts and bacteria.
A long-standing research interest has been the light-harvesting chlorophyll a/b protein complex, LHC-II. After determining the structure of this most abundant of all membrane proteins, first at 3.4 Å resolution by electron crystallography (Kühlbrandt et al, Nature 1994), and then at 2.5 Å resolution by X-ray crystallography (figure 1); Standfuss et al, EMBO J 2005), we established by microspectroscopy that the crystal structure shows the active, energy transmitting state of the complex (Barros et al, EMBO J 2009; Barros and Kühlbrandt, BBA 2009). An important question related to LHC-II investigated by Laura Wilk in collaboration with Jomo Walla in Braunschweig/ Göttingen is the mechanism of controlled annihilation of excitation energy (so-called non-photochemical quenching) in the photosynthetic antenna of green plants. We have designed and tested a model system with LHC-II, the photosystem-II protein PsbS and the carotenoid zeaxanthin reconstituted together into proteoliposomes. With this system we have been able to mimic non-photochemical quenching in vitro for the first time under near-physiological conditions (Wilk et al, PNAS 2013).
Most structures of the individual energy-converting complexes in the mitochondrial inner membrane and chloroplast thylakoids have now been determined by X-ray crystallography. However, to find out how these complexes interact and work together in the cell requires different techniques, especially electron cryo-microscopy (cryo-EM) and electron cryo-tomography (cryo-ET). By single-particle cryo-EM we have determined the structure of a supercomplex of respiratory chain complexes I, III and IV in the inner mitochondrial membrane at ~20 Å resolution (Althoff et al, EMBO J 2012). The map shows how these three complexes interact in the membrane, presumably to avoid the loss of electrons during transfer from NADH to molecular oxygen in the respiratory chain (Figure 2).
Under favourable conditions, large macromolecular assemblies can be visualized directly in the membrane without biochemical isolation. Examining mitochondria and mitochondrial membranes by cryo-ET, we discovered that the ATP synthase (Complex V) is organized into long ribbons of dimers that run along the highly curved ridges of the inner membrane cristae (Figure 4; Strauss et al, EMBO J 2008; Davies et al, PNAS 2011)
Dimer formation depends on the two dimer-specific ATP synthase subunits e and g. Yeast knockout strains that lack these subunits do not have ATP synthase dimers, dimer rows, or lamellar cristae, indicating that the dimer rows are required for cristae formation. Computer simulations in collaboration with José Faraldo-Gomez at this institute suggest that the local curvature imposed on the membrane by the ATP synthase dimer is sufficient for row formation (Figure 7; Davies et al, PNAS 2012)
The electron transport complexes of the respiratory chain, which are excluded from the tightly curved cristae ridges, reside in the membrane regions at either side of the dimer rows. This arrangement suggests that the cristae work as proton conduits or traps, in which the protons are efficiently guided from their source at the respiratory chain supercomplexes to the proton sinks at the dimer rows at the cristae ridges (Figure 8). We are now investigating the link between mitochondrial membrane organization and aging, which may be important for understanding mitochondria-related diseases such as Parkinson's.
Interestingly, the chloroplast ATP synthase in thylakoid membranes does not form dimers and is confined to flat membrane regions at grana end membranes or stroma lamellae, as we have shown by cryo-ET (Daum et al, 2010; 2011) (Figure 9). The different arrangement of the ATP synthase in the two membrane organelles seems to reflect the different pH regime in thylakoids, where the high delta pH between the thylakoid lumen and the stroma makes an elaborate arrangement as in the mitochondrial cristae unnecessary.
Another long-standing research interests in the group is the structure and mechanisms of secondary transporters. Examples are the carnitine transporter CaiT (Figure 10) and the pH-activated sodium-proton antiporters NhaA and NhaP that balance intracellular pH and ion concentration in all living organisms.
NhaA and NhaP represent two related classes of antiporters that show certain structural similarities, but striking differences in terms of pH activation and transport stoichiometry, which must reflect different transport mechanisms. We are especially interested in NhaP because it is similar to the mammalian NHE sodium-proton antiporters, which are important drug targets (Vinothkumar et al, EMBO J 2005; Appel et al, J Mol Biol 2009). We have obtained a 3D map at 7 Å resolution of the low-pH state (Goswami et al, EMBO J 2011), and are now in the process of investigating its pH-induced conformational changes. This work is carried out by Cristina Paulino.
The 3D map of NhaP from Methanococcus jannaschii determined by electron crystallography at 7 Å resolution (Figure 11) indicates that this antiporter has 13 trans-membrane helices, whereas NhaA has 12. The overall structure of the six helices that are thought to harbour the ion translocation site is similar, but the helices at the monomer interface in the NhaP dimer look very different from NhaA (more details...).
Prof. Dr. Werner Kühlbrandt, Direktor
Abteilung für Strukturbiologie
Sekretariat: Monika Hobrack
Tel.: +49 (0) 69 6303-3001
Fax: +49 (0) 69 6303-3002
Forschungsgruppe und Projektsleiter
Ph. D. Students