For the ESRF Structural Biology Group, 2013 has been a year in which we have witnessed the end of an era. The last external user experiment at ID14-4, carried out on 6th December, marked the final closure of the ID14 ‘Quadriga’ suite of beamlines, Europe’s first dedicated undulator-based beamlines for Macromolecular Crystallography (MX), after 15 years of service to the European Structural Biology Community. During its operational lifetime, crystal structures elucidated from single crystal diffraction data collected at ID14 contributed to almost a quarter of the total European-based deposits in the Protein Data Bank. Science performed at the beamline has provided landmark insights in many areas of Biology including the innate immune complement system, large membrane protein complexes such as Photosystems I and II, and the structures and mechanisms of action of viruses. Experiments at ID14, mostly carried out by beamline staff, shed crucial light on the effect of X-ray induced radiation damage on biological macromolecules, paved the way for the use of UV-visible spectroscopy as a tool to ensure the correct interpretation of crystal structures obtained and helped usher in the technique of high throughput crystallography at the ESRF. Without a doubt though, the achievement for which ID14 will probably be best remembered is its role in the solution of the crystal structure of the ribosome which led to a share of the 2009 Nobel Prize in Chemistry for Venki Ramakrishnan of the MRC Laboratory of Molecular Biology (UK) and Ada Yonath of the Weizmann Institute of Science (Israel) - two long-term users of the ESRF. To mark the contribution of ID14 to European Structural Biology, a one-day symposium will be held in conjunction with the 2013 ESRF Users’ Meeting.

The final closure of the ID14 complex aside, 2013 has been a successful and busy year for the beamlines of the Structural Biology Group. The construction and commissioning of the MASSIF facility, ESRF Upgrade Programme project UPBL10, has continued apace: the commissioning of the X-ray optics of both ID30A-1 (MASSIF-1) and ID30A-3 (MASSIF-3) has been completed, the sample environment of MASSIF-1 has begun to take its final shape and we expect these two facilities to take first users in mid- and late-2014, respectively. MASSIF-1 will be a fixed energy (12.7 keV), high intensity end-station offering rapidly variable focal spot sizes (120 μm2 – μ50 m2 for sample evaluation and data collection. MASSIF-3 will be a fixed energy microfocus end-station providing X-ray beam less than 10 μm2 at the sample position. As well as state-of-the art sample changing robotics, both end-stations will be equipped with latest generation pixel detectors (Pilatus3 6M for MASSIF-1, Eiger 4M for MASSIF-3) to ensure the collection of the highest possible quality diffraction data. Progress on ID30B – the ‘replacement’ of ID14-4 – has also been good and here we expect first users towards the end of 2014. The end is thus in sight for the Phase I upgrade of MX facilities at the ESRF. However, our beamlines will continue to evolve. Current plans include the refurbishment of the ID23-2 microfocus facility to produce, in 2016, a (sub)-micrometre sized beam at the sample position. Additionally, we are actively considering proposals to improve the functionality and X-ray beam characteristics at the two MAD beamlines ID23-1 and ID29.

Despite the resources dedicated to the construction and commissioning of UPBL10 and the availability of only four operational MX beamlines (ID14-4, ID23-1, ID23-2 and ID29), output from the Structural Biology Group beamlines has remained high with, at the time of writing, ESRF-based depositions in the Protein Data Bank for 2013 being close to record levels. An annual refrain is that the articles presented in the Structural Biology Highlights chapter represent only a very small part of the Science reported by the ESRF’s external Structural Biology User Community. This remains true for 2013 but we hope that the work presented here provides a true representation of the broad range of research facilitated by our beamlines. Particularly noteworthy are two reports on membrane proteins. The first, by Neutze and colleagues, describes the crystal structure of an aquaporin - involved in cellular water transport – at subangstrom resolution (dmin = 0.88 Å). This represents the first truly atomic resolution study of a membrane protein. Here, electron density and difference density maps of unparalleled quality have allowed the unambiguous assignment of both orientation and tautomeric form of amino acid residues crucial to the specificity of aquaporins, and this study has provided true atomic level insight into the control of water homeostasis in living cells. The second, by Sazanov and colleagues, reports the crystal structure of the entire respiratory complex I from T. thermophilus at 3.3 Å resolution. With a total molecular mass of 536 kDa and comprising 16 subunits, 9 Fe-S clusters and 64 transmembrane helices, this crystal structure represents the largest asymmetric membrane protein structure thus far solved and suggests a likely mechanism, involving long range conformational changes, for the coupling of electron transfer in the hydrophilic domain to proton translocation in the membrane domain of complex I.

G. Leonard