Protein biological function depends on structural flexibility and change. From cellular communication through membrane ion channels to oxygen uptake and delivery by haemoglobin, structural changes are critical. It has been suggested that vibrations that extend through the protein play a crucial role in controlling these structural changes. While nature may utilize such long-range vibrations for optimization of biological processes, bench-top characterization of these extended structural motions for engineered biochemistry has been elusive. Here we show the first optical observation of long-range protein vibrational modes. This is achieved by orientation-sensitive terahertz near-field microscopy measurements of chicken egg white lysozyme single crystals. Underdamped modes are found to exist for frequencies 410 cm À 1 . The existence of these persisting motions indicates that damping and intermode coupling are weaker than previously assumed. The methodology developed permits protein engineering based on dynamical network optimization.

Part of the challenge of macromolecular crystal growth for structure determination is obtaining crystals with a volume suitable for x-ray analysis. In this respect an understanding of the effect of solution conditions on macromolecule nucleation rates is advantageous. This study investigated the effects of supersaturation, temperature, and pH on the nucleation rate of tetragonal lysozyme crystals. Batch crystallization plates were prepared at given solution concentrations and incubated at set temperatures over 1 week. The number of crystals per well with their size and axial ratios were recorded and correlated with solution conditions. Crystal numbers were found to increase with increasing supersaturation and temperature. The most significant variable, however, was pH; crystal numbers changed by two orders of magnitude over the pH range 4.0-5.2. Crystal size also varied with solution conditions, with the largest crystals obtained at pH 5.2. Having optimized the crystallization conditions, we prepared a batch of crystals under the same initial conditions, and 50 of these crystals were analyzed by x-ray diffraction techniques. The results indicate that even under the same crystallization conditions, a marked variation in crystal properties exists.

Macromolecular crystal growth is seen as an ideal experiment to make use of the reduced acceleration environment provided by an orbiting spacecraft. The experiments are small, are simply operated, and have a high potential scientific and economic impact. In this review we examine the theoretical reasons why microgravity is a beneficial environment for crystal growth and survey the history of experiments on the Space Shuttle Orbiter, on unmanned spacecraft, and on the Mir space station. The results of microgravity crystal growth are considerable when one realizes that the comparisons are always between few microgravity-based experiments and a large number of earth-based experiments. Finally, we outline the direction for optimizing the future use of orbiting platforms.

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SUMMARY Sliding clamp proteins topologically encircle DNA and play vital roles in coordinating the actions of various DNA replication, repair, and damage tolerance proteins. At least three distinct surfaces of the E. coli β clamp interact physically with the DNA that it topologically encircles. We utilized mutant β clamp proteins bearing G66E and G174A substitutions (β159), affecting the single strand (ss) DNA-binding region, or poly-Ala substitutions in place of residues 148-HQDVR-152 (β148–152), affecting the double strand (ds) DNA binding region, in order to determine the biological relevance of clamp-DNA interactions. As part of this work, we solved the x-ray crystal structure of β148–152, which verified that the poly-Ala substitutions failed to significantly alter the tertiary structure of the clamp. Based on functional assays, both β159 and β148–152 were impaired for loading and retention on a linear primed DNA in vitro. In the case of β148–152, this defect was not due to altered interactions with the DnaX clamp loader, but rather was the result of impaired β148–152-DNA interactions. Once loaded, β148–152 was proficient for Pol III replication in vitro. In contrast, β148–152 was severely impaired for Pol II and Pol IV replication, and was similarly impaired for direct physical interactions with these Pols. Despite its ability to support Pol III replication in vitro, β148–152 was unable to support viability of E. coli. Nevertheless, physiological levels of β148–152 expressed from a plasmid efficiently complemented the temperature sensitive growth phenotype of a strain expressing β159 (dnaN159), provided that Pol II and Pol IV were inactivated. Although this strain was impaired for Pol V-dependent mutagenesis, inactivation of Pol II and Pol IV restored the Pol V mutator phenotype. Taken together, these results support a model in which a sophisticated combination of competitive clamp-DNA, clamp-partner, and partner-DNA interactions serve to manage the actions of the different E. coli Pols in vivo.

Density difference fluid flows and sedimentation of growing crystals are greatly reduced when crystallization takes place in a reduced gravity environment. In the case of macromolecular crystallography a crystal of a biological macromolecule is used for diffraction experiments (x-ray or neutron) so as to determine the three-dimensional structure of the macromolecule. The better the internal order of the crystal then the greater the molecular structure detail that can be extracted. It is this structural information that enables an understanding of how the molecule functions. This knowledge is changing the biological and chemical sciences, with major potential in understanding disease pathologies.In this review, we examine the use of microgravity as an environment to grow macromolecular crystals. We describe the crystallization procedures used on the ground, how the resulting crystals are studied and the knowledge obtained from those crystals. We address the features desired in an ordered crystal and the techniques used to evaluate those features in detail. We then introduce the microgravity environment, the techniques to access that environment and the theory and evidence behind the use of microgravity for crystallization experiments. We describe how ground-based laboratory techniques have been adapted to microgravity flights and look at some of the methods used to analyse the resulting data. Several case studies illustrate the physical crystal quality improvements and the macromolecular structural advances. Finally, limitations and alternatives to microgravity and future directions for this research are covered.Macromolecular structural crystallography in general is a remarkable field where physics, biology, chemistry and mathematics meet to enable insight to the fundamentals of life. As the reader will see, there is a great deal of physics involved when the microgravity environment is applied to crystallization, some of it known, and undoubtedly much yet to discover.

Microgravity offers an environment for protein crystallization where there is an absence of convection and sedimentation. We have investigated the effect of microgravity conditions on the perfection of protein crystals. The quality of crystals for X‐ray diffraction studies is characterized by a number of factors, namely size, mosaicity and the resolution limit. By using tetragonal lysozyme crystals as a test case we show, with crystal growth in two separate Space Shuttle missions, that the mosaicity is improved by a factor of three to four over earth‐grown ground control values. These microgravity‐grown protein crystals are then essentially perfect diffraction gratings. As a result the peak to background of individual X‐ray diffraction reflections is enhanced by a similar factor to the reduction in the mosaicity. This then offers a particularly important opportunity for improving the measurement of weak reflections such as occur at high diffraction resolution. These microgravity results set a benchmark for all future microgravity and earth‐based protein crystallography procedures.

A comprehensive study of microgravity and ground-grown chicken egg-white lysozyme crystals is presented using synchrotron X-ray reciprocal-space mapping, topography techniques and diffraction resolution. Microgravity crystals displayed reduced intrinsic mosaicities on average, but no differences in terms of strain over their ground-grown counterparts. Topographic analysis revealed that in the microgravity case the majority of the crystal was contributing to the peak of the reflection at the appropriate Bragg angle. In the ground-control case only a small volume of the crystal contributed to the intensity at the diffraction peak. The techniques prove to be highly complementary, with the reciprocal-space mapping providing a quantitative measure of the crystal mosaicity and strain (or variation in lattice spacing) and the topography providing a qualitative overall assessment of the crystal in terms of its X-ray diffraction properties. Structural data collection was also carried out at the synchrotron.