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The Triumph and despair of protein crystallization in space.   J. D. Ng¹ and R. Giegй². ¹Laboratory for Structural Biology and the Department of Biological Sciences, University of Alabama in Huntsville, Huntsville, AL 35899 USA ²UPR 9002, IBMC du CNRS, 15 rue Renй Descartes, 67084 Strasbourg Cedex, FRANCE

   Over two decades of protein crystallization experiments have been performed under microgravity aboard the Space Shuttle and the International Space Station.  Even though there has been a great deal of skepticism on the value of growing protein crystals in space, there is much evidence that crystals of improved quality can be obtained under microgravity compared to those grown on Earth.  The history of protein crystal growth and the methods that have been used to assess protein crystal quality will be reviewed.  The quality of protein crystals grown under microgravity have been compared to those of Earth-grown by directly visualizing crystalline habit, analyzing scattered X-ray intensity of reflections over their background noise, measuring mosaicity and determining the crystal’s ability to diffract X-rays sufficiently for ab initio structure determination without any a priori structural information.  Modeled proteins of different size and function have been shown to crystallize better in space and become more useful for structure determination.  In the case of some proteins, the potential for a crystal to provide accurate phase information for de novo structure determination is higher for space-grown crystals compared to those from Earth.  In the frame of structural biology, there has been much apprehension on the cost-effectiveness and efficacy of using microgravity to produce protein crystals for biological applications.  We evaluate the overall space program for macromolecular crystal growth and discuss the lessons learned from over 20 years of protein crystallization in space.