Explore the vast range of titles available.

A variety of organisms have evolved mechanisms to detect and respond to light, in which the response is mediated by protein structural changes after photon absorption. The initial step is often the photoisomerization of a conjugated chromophore. Isomerization occurs on ultrafast time scales and is substantially influenced by the chromophore environment. Here we identify structural changes associated with the earliest steps in the trans-to-cis isomerization of the chromophore in photoactive yellow protein. Femtosecond hard x-ray pulses emitted by the Linac Coherent Light Source were used to conduct time-resolved serial femtosecond crystallography on photoactive yellow protein microcrystals over a time range from 100 femtoseconds to 3 picoseconds to determine the structural dynamics of the photoisomerization reaction.

G-protein-coupled receptors (GPCRs) signal primarily through G proteins or arrestins. Arrestin binding to GPCRs blocks G protein interaction and redirects signalling to numerous G-protein-independent pathways. Here we report the crystal structure of a constitutively active form of human rhodopsin bound to a pre-activated form of the mouse visual arrestin, determined by serial femtosecond X-ray laser crystallography. Together with extensive biochemical and mutagenesis data, the structure reveals an overall architecture of the rhodopsin–arrestin assembly in which rhodopsin uses distinct structural elements, including transmembrane helix 7 and helix 8, to recruit arrestin. Correspondingly, arrestin adopts the pre-activated conformation, with a ∼20° rotation between the amino and carboxy domains, which opens up a cleft in arrestin to accommodate a short helix formed by the second intracellular loop of rhodopsin. This structure provides a basis for understanding GPCR-mediated arrestin-biased signalling and demonstrates the power of X-ray lasers for advancing the frontiers of structural biology. G protein-coupled receptors are a large family of signalling proteins that mediate cellular responses primarily via G proteins or arrestins, and they are targets of one-third of the current clinically used drugs; here, an active form of human rhodopsin bound to a pre-activated form of the mouse visual arrestin-1 is determined, revealing unique structural features that may constitute essential elements for arrestin-biased signalling. Arrestin binding to GPCRs The ubiquitous signalling proteins known as G-protein-coupled receptors (GPCRs) signal primarily through G proteins or arrestins, and they are targets of one-third of the current clinically used drugs. This paper presents the first crystal structure of a GPCR bound to arrestin — an active form of human rhodopsin bound to a pre-activated form of the mouse visual arrestin-1 — based on X-ray crystallography, electron microscopy, pulsed EPR spectroscopy and hydrogen-deuterium exchange mass spectrometry. Comparison of the structure to those of the G-protein-bound β 2 AR and the GαCT-bound rhodopsin reveal unique structural features that may constitute essential elements for arrestin-biased signalling.

Serial femtosecond crystallography using ultrashort pulses from x-ray free electron lasers (XFELs) enables studies of the light-triggered dynamics of biomolecuies. We used microcrystals of photoactive yellow protein (a bacterial blue light photoreceptor) as a model system and obtained high-resolution, time-resolved difference electron density maps of excellent quality with strong features; these allowed the determination of structures of reaction intermediates to a resolution of 1.6 angstroms. Our results open the way to the study of reversible and nonreversible biological reactions on time scales as short as femtoseconds under conditions that maximize the extent of reaction initiation throughout the crystal.

Femtosecond X-ray pulses were used to obtain diffraction data on photosystem II, revealing conformational changes as the complex transitions from the dark S 1 state to the double-pumped S 3 state; the time-resolved serial femtosecond crystallography technique enables structural determination of protein conformations that are highly prone to traditional radiation damage. An X-ray snapshot of photosystem II structure It has recently been shown that extremely short and intense radiation pulses from X-ray free-electron lasers can be used to obtain diffraction data on nanometre- to micrometre-sized protein crystals before the crystal suffers radiation damage. The hope is that this 'serial femtosecond crystallography' (SFX) approach will produce structures of proteins and protein complexes that do not yield well-ordered macroscopic crystals. These authors collected time-resolved SFX data on small crystals of photosystem II of photosynthesis during its transition from the 'dark' S1 state to the double-excited S3 state. At present the resolution of this technique is moderate, but it is sufficient to reveal significant conformational changes at the Mn 4 CaO 5 cluster at the heart of the oxygen evolving complex and at the electron acceptor site. Photosynthesis, a process catalysed by plants, algae and cyanobacteria converts sunlight to energy thus sustaining all higher life on Earth. Two large membrane protein complexes, photosystem I and II (PSI and PSII), act in series to catalyse the light-driven reactions in photosynthesis. PSII catalyses the light-driven water splitting process, which maintains the Earth’s oxygenic atmosphere 1 . In this process, the oxygen-evolving complex (OEC) of PSII cycles through five states, S 0 to S 4 , in which four electrons are sequentially extracted from the OEC in four light-driven charge-separation events. Here we describe time resolved experiments on PSII nano/microcrystals from Thermosynechococcus elongatus performed with the recently developed 2 technique of serial femtosecond crystallography. Structures have been determined from PSII in the dark S 1 state and after double laser excitation (putative S 3 state) at 5 and 5.5 Å resolution, respectively. The results provide evidence that PSII undergoes significant conformational changes at the electron acceptor side and at the Mn 4 CaO 5 core of the OEC. These include an elongation of the metal cluster, accompanied by changes in the protein environment, which could allow for binding of the second substrate water molecule between the more distant protruding Mn (referred to as the ‘dangler’ Mn) and the Mn 3 CaO x cubane in the S 2 to S 3 transition, as predicted by spectroscopic and computational studies 3 , 4 . This work shows the great potential for time-resolved serial femtosecond crystallography for investigation of catalytic processes in biomolecules.

Photosynthetic organisms have adapted to survive a myriad of extreme environments from the earth’s deserts to its poles, yet the proteins that carry out the light reactions of photosynthesis are highly conserved from the cyanobacteria to modern day crops. To investigate adaptations of the photosynthetic machinery in cyanobacteria to excessive light stress, we isolated a new strain of cyanobacteria, Cyanobacterium aponinum 0216, from the extreme light environment of the Sonoran Desert. Here we report the biochemical characterization and the 2.7 Å resolution structure of trimeric photosystem I from this high-light-tolerant cyanobacterium. The structure shows a new conformation of the PsaL C-terminus that supports trimer formation of cyanobacterial photosystem I. The spectroscopic analysis of this photosystem I revealed a decrease in far-red absorption, which is attributed to a decrease in the number of long- wavelength chlorophylls. Using these findings, we constructed two chimeric PSIs in Synechocystis sp. PCC 6803 demonstrating how unique structural features in photosynthetic complexes can change spectroscopic properties, allowing organisms to thrive under different environmental stresses.

X-ray crystallography of G protein-coupled receptors and other membrane proteins is hampered by difficulties associated with growing sufficiently large crystals that withstand radiation damage and yield high-resolution data at synchrotron sources. We used an x-ray free-electron laser (XFEL) with individual 50-femtosecond-duration x-ray pulses to minimize radiation damage and obtained a high-resolution room-temperature structure of a human serotonin receptor using sub-10-micrometer microcrystals grown in a membrane mimetic matrix known as lipidie cubic phase. Compared with the structure solved by using traditional microcrystallography from cryo-cooled crystals of about two orders of magnitude larger volume, the room-temperature XFEL structure displays a distinct distribution of thermal motions and conformations of residues that likely more accurately represent the receptor structure and dynamics in a cellular environment.

Background Ever since the first atomic structure of an enzyme was solved, the discovery of the mechanism and dynamics of reactions catalyzed by biomolecules has been the key goal for the understanding of the molecular processes that drive life on earth. Despite a large number of successful methods for trapping reaction intermediates, the direct observation of an ongoing reaction has been possible only in rare and exceptional cases. Results Here, we demonstrate a general method for capturing enzyme catalysis “in action” by mix-and-inject serial crystallography (MISC) . Specifically, we follow the catalytic reaction of the Mycobacterium tuberculosis β-lactamase with the third-generation antibiotic ceftriaxone by time-resolved serial femtosecond crystallography. The results reveal, in near atomic detail, antibiotic cleavage and inactivation from 30 ms to 2 s. Conclusions MISC is a versatile and generally applicable method to investigate reactions of biological macromolecules, some of which are of immense biological significance and might be, in addition, important targets for structure-based drug design. With megahertz X-ray pulse rates expected at the Linac Coherent Light Source II and the European X-ray free-electron laser, multiple, finely spaced time delays can be collected rapidly, allowing a comprehensive description of biomolecular reactions in terms of structure and kinetics from the same set of X-ray data.

Lipidic cubic phase (LCP) crystallization has proven successful for high-resolution structure determination of challenging membrane proteins. Here we present a technique for extruding gel-like LCP with embedded membrane protein microcrystals, providing a continuously renewed source of material for serial femtosecond crystallography. Data collected from sub-10-μm-sized crystals produced with less than 0.5 mg of purified protein yield structural insights regarding cyclopamine binding to the Smoothened receptor. Serial femtosecond X-ray crystallography permits the use of very small protein crystals; however, a continuous flow of sample is required. Weierstall et al . design and demonstrate an injector system that can supply microcrystals in the lipidic cubic phase, dramatically reducing the quantities of protein required.

Structure determination of proteins and other macromolecules has historically required the growth of high-quality crystals sufficiently large to diffract x-rays efficiently while withstanding radiation damage. We applied serial femtosecond crystallography (SFX) using an x-ray free-electron laser (XFEL) to obtain high-resolution structural information from microcrystals (less than 1 micrometer by 1 micrometer by 3 micrometers) of the well-characterized model protein lysozyme. The agreement with synchrotron data demonstrates the immediate relevance of SFX for analyzing the structure of the large group of difficult-to-crystallize molecules.

Ultrapurified Photosystem II complexes crystalize as uniform microcrystals (PSIIX) of unprecedented homogeneity that allow observation of details previously unachievable, including the longest sustained oscillations of flash-induced O 2 yield over > 200 flashes and a novel period-4.7 water oxidation cycle. We provide new evidence for a molecular-based mechanism for PSII-cyclic electron flow that accounts for switching from linear to cyclic electron flow within PSII as the downstream PQ/PQH 2 pool reduces in response to metabolic needs and environmental input. The model is supported by flash oximetry of PSIIX as the LEF/CEF switch occurs, Fourier analysis of O 2 flash yields, and Joliot-Kok modeling. The LEF/CEF switch rebalances the ratio of reductant energy (PQH 2 ) to proton gradient energy (H + o /H + i ) created by PSII photochemistry. Central to this model is the requirement for a regulatory site (Q C ) with two redox states in equilibrium with the dissociable secondary electron carrier site Q B . Both sites are controlled by electrons and protons. Our evidence fits historical LEF models wherein light-driven water oxidation delivers electrons (from Q A − ) and stromal protons through Q B to generate plastoquinol, the terminal product of PSII-LEF in vivo. The new insight is the essential regulatory role of Q C . This site senses both the proton gradient (H + o /H + i ) and the PQ pool redox poise via e − /H + equilibration with Q B . This information directs switching to CEF upon population of the protonated semiquinone in the Qc site (Q − H + ) C , while the WOC is in the reducible S2 or S3 states. Subsequent photochemical primary charge separation (P + Q A − ) forms no (QH 2 ) B , but instead undergoes two-electron backward transition in which the Q C protons are pumped into the lumen, while the electrons return to the WOC forming (S1/S2). PSII-CEF enables production of additional ATP needed to power cellular processes including the terminal carboxylation reaction and in some cases PSI-dependent CEF.