Compartmentalization of biochemical reactions is the basis for the development of life. Biological membranes delineate cells and subcellular compartments to control different cellular processes. However, these compartments cannot function in isolation; they must transfer information and matter, including ions, metabolites, proteins, and harmful compounds, across the membrane barrier. Cell membranes are therefore equipped with a variety of membrane proteins encoded by about one-third of the cell genome. These transporters, channels, receptors and enzymes facilitate transport and information exchange across this barrier.

 

The goal of the Collaborative Research Center CRC 807 (2008 - 2020) was to elucidate the structure, function, and mechanism of membrane proteins involved in the transfer of matter and information across cell membranes. Research interests ranged from small functional units to large, highly dynamic assemblies of multiple subunits in subcellular compartments. Methods included X-ray crystallography, cryo-electron microscopy, solid-state and solution NMR, pulsed EPR, time-resolved visible and infrared spectroscopy, single-molecule fluorescence techniques, super-resolution microscopy, native mass spectrometry, electrophysiology, and computational biophysics. Members of the CRC 807 conducted research on integrative biological problems, both applying and developing advanced methods. The CRC relied on a balanced combination of exciting topics, current issues, and new approaches under five subtopics:

 

(i) secondary active transporters

Trimer architecture of BetP. a, b, The BetP trimer as seen from the periplasmic (a) and cytosolic (b) sides of the membrane. c, View into the cleft between monomers A and C. d, Surface representation of the trimer oriented as in c. The negative to positive electrostatic surface is coloured red to blue, respectively (Ressl et al. 2019 Nature).

 

(ii) ABC transporters

The conformational space of a heterodimeric ABC transport complex. a, d, Distinct IF (a) and OF (d) conformations of TmrAB in lipid environment were observed under turnover conditions. b, In the ATP-bound state, the NBDs are tightly dimerized and the exporter exhibits either an OFopen or an OFoccluded conformation. c, OFopen and OFoccluded conformations also dominate in the vanadate-trapped state (Hofmann et al. 2019 Nature).

 

(iii) 7TM receptors and retinal proteins

General structure presentation of ChR2. A, Four cavities and three gates forming the channel pore. B, Extended hydrogen-bond network. The DC gate is shown in the red ellipse. The black arrows and gray horizontal lines show the putative ion pathway and position of hydrophobic/hydrophilic boundaries, respectively (Volkov et al. 2017 Science).

 

(iv) rotating ATPases

Cryo-EM structure of the Polytomella ATP synthase dimer. The F1 head (green) is linked to the c-ring rotor (yellow) by the central stalk and the peripheral stalk. Insets (beginning at top right) show the flexible OSCP hinge (orange); F1 rotary substates with subunits b (green), g (blue), and c (yellow); a coordinated metal ion in the proton access channel (light blue); and the dimer-forming subunit ASA10 (red) (Murphy et al. 2019 Science).

 

(v) membrane complexes

Structure of the MHC I peptide-loading complex (PLC). a, The fully assembled PLC comprises two editing modules that are arranged around the central antigen translocation machinery TAP1/2. Each editing module consists of the scaffold chaperone calreticulin (Crt), the oxidoreductase ERp57, the MHC I-specific chaperone tapasin (Tsn), and the MHC I heterodimer of heavy chain (hc) and β₂-microglobulin (β₂m). b, View from the ER lumen onto the PLC. The two Tsn molecules form the central lumenal framework, while Crt is anchored through its lectin domain to the N-linked glycan of the MHC I hc and interacts with ERp57 via the tip of its arm-like proline-rich (P)-domain (Blees et al. 2017 Nature). d, Assembly and disassembly of the PLC, highlighting the molecular sociology between a translocation machinery and the ER quality control network (Thomas & Tampé 2020 Curr Opin Immunol). c, X-ray structure of the MHC I-TAPBPR chaperone complex (Thomas & Tampé 2017 Science).

 

The CRC 807 has been extremely successful in applying a broad range of biochemical and biophysical techniques to elucidate transmembrane processes in mechanistic detail with high spatial and temporal resolution. The different sequence of events during a transfer cycle, their time scales, and their structural bases have been elucidated. The conformational states and interaction sites of transporters and transmembrane receptors were identified and characterized, paving the way to understanding these processes and helping to unlock these insights for appropriate therapeutic interventions. Over the last 12 years, the CRC has been part of a transformation. Starting with a sparsely populated database, more than 1,000 unique membrane protein structures have been deposited since then.

 

The following scientists were involved in the SFB (in alphabetical order):

 

 

The following scientists were involved in the CRC 807 through associated projects:

 

José Faraldo-Gómez
Misha Kudryashev
Arne Möller
Volker Zickermann