Search

Two-component signal transduction is the major signaling mechanism that enables bacteria to survive and thrive in complex environmental conditions. The photosynthetic bacterium R. palustris employs two tandem bacteriophytochromes, RpBphP2 and RpBphP3, to perceive red-light signals that regulate the synthesis of light-harvesting complexes under low-light conditions. Both RpBphP2 and RpBphP3 are photosensory histidine kinases coupled to the same response regulator RPA3017. Together, they constitute a two-component system that converts a red-light signal into a biological signal. In this work, the crystal structure of RPA3017 in the unphosphorylated form at 1.9 Å resolution is presented. This structure reveals a tightly associated dimer arrangement that is conserved among phytochrome-related response regulators. The conserved active-site architecture provides structural insight into the phosphotransfer reaction between RpBphP2/RpBphP3 and RPA3017. Based on structural comparisons and homology modeling, how specific recognition between RpBphP2/RpBphP3 and RPA3017 is achieved at the molecular level is further explored. [ABSTRACT FROM AUTHOR]

A key role in signal transduction and dimerization mediated by Per-Arnt-Sim (PAS) domains is played by α-helical linkers that flank the structurally similar α/β cores of these domains. However, crystal-packing forces and the different construct lengths and sequences of the PAS domains influence the final length and orientation of the linkers relative to the core and create uncertainty in the exact mechanism of the linker function. Thus, structural characterization and comparison of the linkers within isolated PAS-domain constructs and/or full-length PAS-containing proteins is important for clarification of the mechanism. The plant blue-light photoreceptors phototropins possess two N-terminal flavin mononucleotide-based light, oxygen or voltage (LOV) domains (LOV1 and LOV2) that comprise a subclass of the PAS family and one C-terminal serine/threonine kinase domain whose enzymatic activity is regulated by blue light. The dark-adapted state crystal structures of the Arabidopsis thaliana phototropin 1 and phototropin 2 LOV1-domain constructs flanked by an N-terminal A′α helix and the structure of the phototropin 2 core LOV2 domain are known. Here, the crystal structure of the A. thaliana phototropin 1 LOV2 domain has been determined in its dark-adapted state. The core is flanked by an N-terminal A′α helix and a C-terminal Jα helix similar to those in the previously reported structure of Avena sativa phototropin 1 LOV2. In contrast to the monomeric A. sativa LOV2, A. thaliana LOV2 is a dimer in which two A′α helices adopt a scissor-like orientation at the dimer interface and form a short α-helical coiled coil. The J α helix predominantly interacts with the β-sheet and plays a role in coiled-coil formation and dimerization. [ABSTRACT FROM AUTHOR]

Dynamic behavior of proteins is critical to their function. X-ray crystallography, a powerful yet mostly static technique, faces inherent challenges in acquiring dynamic information despite decades of effort. Dynamic `structural changes' are often indirectly inferred from `structural differences' by comparing related static structures. In contrast, the direct observation of dynamic structural changes requires the initiation of a biochemical reaction or process in a crystal. Both the direct and the indirect approaches share a common challenge in analysis: how to interpret the structural heterogeneity intrinsic to all dynamic processes. This paper presents a real-space approach to this challenge, in which a suite of analytical methods and tools to identify and refine the mixed structural species present in multiple crystallographic data sets have been developed. These methods have been applied to representative scenarios in dynamic crystallography, and reveal structural information that is otherwise difficult to interpret or inaccessible using conventional methods. [ABSTRACT FROM AUTHOR]

The phototropins are blue-light receptors that base their light-dependent action on the reversible formation of a covalent bond between a flavin mononucleotide (FMN) cofactor and a conserved cysteine in light, oxygen or voltage (LOV) domains. The primary reactions of the Avena sativa phototropin 1 LOV2 domain were investigated by means of time-resolved and low-temperature fluorescence spectroscopy. Synchroscan streak camera experiments revealed a fluorescence lifetime of 2.2 ns in LOV2. A weak long-lived component with emission intensity from 600 to 650 nm was assigned to phosphorescence from the reactive FMN triplet state. This observation allowed determination of the LOV2 triplet state energy level at physiological temperature at 16600 cm. FMN dissolved in aqueous solution showed pH-dependent fluorescence lifetimes of 2.7 ns at pH 2 and 3.9−4.1 ns at pH 3-8. Here, too, a weak phosphorescence band was observed. The fluorescence quantum yield of LOV2 increased from 0.13 to 0.41 upon cooling the sample from 293 to 77 K. A pronounced phosphorescence emission around 600 nm was observed in the LOV2 domain between 77 and 120 K in the steady-state emission. [ABSTRACT FROM AUTHOR]

Eight high-resolution crystal structures of the ground state of photoactive yellow protein (PYP) solved under a variety of conditions reveal that its chromophore is stabilized by two unusually short hydrogen bonds. Both Tyr42 On and Glu46 Oε are separated from the chromophore phenolate oxygen by less than the sum of their atomic van der Waals radii, 2.6 Å. This is characteristic of strong hydrogen bonding, in which hydrogen bonds acquire significant covalent character. The hydrogen bond from the protonated Glu46 to the negatively charged phenolate oxygen is 2.58 ± 0.01 Å in length, while that from Tyr42 is considerably shorter, 2.49 ± 0.01 Å. The E46Q mutant was solved to 0.95 Å resolution; the isosteric mutation increased the length of the hydrogen bond from Glx46 to the chromophore by 0.29 ± 0.01 Å to that of an average hydrogen bond, 2.88 ± 0.01 Å. The very short hydrogen bond from Tyr42 explains why mutating this residue has such a severe effect on the ground-state structure and PYP photocycle. The effect of isosteric mutations on the photocycle can be largely explained by the alterations to the length and strength of these hydrogen bonds. [ABSTRACT FROM AUTHOR]

Time-resolved crystallography has been successfully applied on the time scale from seconds via milliseconds and nanoseconds to picoseconds on a variety of systems, This brief review largely deals with macromolecular systems, on which there has been substantial recent progress. The strategies for design of a successful experiment that eliminates or minimizes potential artefacts have been identified, and the specifically crystallographic components of these strategies have been implemented. The remaining computational challenge is to identify and extract time-independent structures, each corresponding to a distinct reaction intermediate, whose populations vary with time and give rise to the time-dependent X-ray diffraction data. The fourth dimension, time, has been added to the three spatial dimensions of crystallography; it can no longer be regarded as purely a static technique. [ABSTRACT FROM AUTHOR]

Singular value decomposition (SVD) separates time-dependent crystallographic data into time-independent and time-dependent components. Procedures for the effective application of SVD to time-resolved macromolecular crystallographic data have yet to be explored systematically. Here, the applicability of SVD to experimental crystallographic data is tested by analyzing 30 time-resolved Laue data sets spanning a time range of nanoseconds to milliseconds through the photocycle of the E46Q mutant of photoactive yellow protein. The data contain random and substantial systematic errors, the latter largely arising from crystal-to-crystal variation. The signal-to-noise ratio of weighted difference electron-density maps is significantly improved by the SVD flattening procedure. Application of SVD to these flattened maps spreads the signal across many of the 30 singular vectors, but a rotation of the vectors partitions the large majority of the signal into only five singular vectors. Fitting the time-dependent vectors to a sum of simple exponentials suggests that a chemical kinetic mechanism can describe the time-dependent structural data. Procedures for the effective SVD analysis of experimental time-resolved crystallographic data have been established and emphasize the necessity for minimizing systematic errors by modification of the data-collection protocol.