Abstract
The Japan Aerospace Exploration Agency (JAXA) started a high-quality protein crystal growth project, now called JAXA PCG, on the International Space Station (ISS) in 2002. Using the counter-diffusion technique, 14 sessions of experiments have been performed as of 2012 with 580 proteins crystallized in total. Over the course of these experiments, a user-friendly interface framework for high accessibility has been constructed and crystallization techniques improved; devices to maximize the use of the microgravity environment have been designed, resulting in some high-resolution crystal growth. If crystallization conditions were carefully fixed in ground-based experiments, high-quality protein crystals grew in microgravity in many experiments on the ISS, especially when a highly homogeneous protein sample and a viscous crystallization solution were employed. In this article, the current status of JAXA PCG is discussed, and a rational approach to high-quality protein crystal growth in microgravity based on numerical analyses is explained.
Highlights
With regard to the X-ray diffraction analysis of protein crystals, the technique has been progressing over the last decades, from protein sample preparations and optimization of crystallization conditions to improvements in hardware and software for X-ray diffraction experiments and beyond
In 2000, the National Research Council of the USA reported that the impact of microgravity crystallization on structural biology as a whole is extremely limited before fully understanding the differences in quality between microgravity- and ground-grown crystals
A ‘good result’ means that the quality of the crystal grown in space, indexed by maximum resolution, mosaicity or improved morphology, was higher than that of the best ground-grown crystal which was grown with the usual vapor-diffusion method or the counter-diffusion method as a ground-control experiment, and the X-ray diffraction data set advanced the research of the users
Summary
With regard to the X-ray diffraction analysis of protein crystals, the technique has been progressing over the last decades, from protein sample preparations and optimization of crystallization conditions to improvements in hardware and software for X-ray diffraction experiments and beyond. On the other hand, Snell & Helliwell (2005) noted that the success rate of space experiments based on the criteria of improved diffraction resolution increased to 60% when the analysis was limited to proteins that have flown four or more times This implies that microgravity would provide an advantage in obtaining better-quality crystals if the crystallization technique was well optimized. JAXA has aimed to regularly obtain high-quality crystals of useful proteins, so that microgravity crystallization can become one of the choices for researchers to consider in their experiment designs For this purpose, a standard user-friendly experiment protocol was established to expedite the process from the acceptance of the samples to the launch. Technical achievements in high-quality crystal growth and upgrades in the support system for users continue to advance, and, at present, if crystallization conditions are properly optimized, the reproducibility of high-quality crystal growth is a likely expectation
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