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Protein crystallography Bernal, Klug emphasized the importance of flexible thinking and how Bernal's aphorism, 'crystallographers must learn not Fifty years of pepsin crystals to be crystallographers', applied over and over again. This was as true in Pauling's jrom Guy Dodson and Cyrus Chothia production of the a-helix structure and in Bernal's approach to the water problem as IN the middle of April 1934, J.D. Bernal The crystals failed to diffract after removal it is now in the study of the behaviour of and Dorothy Crowfoot (now Hodgkin) from the test tube and drying but, crucially, protein structure which is increasingly obtained the first successful X-ray photo­ Bernal noticed that their birefringence had found to link well defined domains to flex­ graphs of a globular protein. They were also disappeared. Thus he realized the need ible and variable segments. Thus, the link­ obtained in Cambridge from crystals of to keep crystals wet. Bernal's solution was ing of order and disorder to achieve pepsin grown by John Philpot in Uppsala. to put the crystals in capillaries (made assembly and function (RNA binding) in The photographs, discussed in a letter to possible because of Helen Megaw's studies the tobacco mosaic virus is revealed by Nature (133, 794; 1934) published half a on ice crystals). The subsequent investiga­ crystallographic analysis. A report by century ago, almost to the day, were the tion of the crystal's diffraction charac­ Steven Harrison (Harvard University) of start of one of the great achievements of teristics, cell dimensions and space group the 2.8 A resolution crystal structure of modern biology. was carried out by Bernal and Dorothy tomato bushy stunt virus nicely completed On 13 April 1984 a meeting was held in Crowfoot. It led to their recognition "that studies begun by Bernal and Fankuchen. Cambridge to commemorate the fiftieth the arrangement of atoms inside the pro­ This virus also demonstrates beautifully anniversary of the experiment. Lectures tein molecule is of a perfectly definite the order-disorder phenomenon, which were given by Dorothy Hodgkin, Max kind" and to remarkably accurate ideas Harrison showed is related to the packing Perutz and Aaron Klug, all of whom about the molecules' molecUlar weight and and assembly of the virus subunits. worked at different times in Bernal's shape. There are many conclusions from a day laboratory. The occasion brought together The descriptions of the subsequent like this. First, there are the remarkably all those still living who were directly in­ research into proteip crystals included the happy collaborative traditions in protein volved in the pepsin experiment; sadly, details of how the studies on insulin, crystallography, established from the earli­ Peter Wooster, a long-established figure in haemoglobin and the viruses began. The est days, that make it such fun to work in Bernal's laboratory and remembered immense difficulties in solving these struc­ the field. Second, the techniques, essential­ warmly by many, died on the night pre­ tures were clearly appreciated aUhe time ly physics, have put the description of pro­ ceding the meeting. Several crystallo­ and the early investigations were devoted tein and biomolecular structure into the graphers who had contributed to the field, to measuring X-ray intensities, to calculat­ hands of crystallographers. Finally, the im­ notably Arthur Wilson (intensity statistics) ing the Patterson function (also celebrating portance of crystals and other ordered and Henry Lipson (Fourier calculation its fiftieth anniversary this year) and to systems as a route to the description of bio­ methods), were also present. characterizing new protein crystals. The in­ chemical and biological structure remains Part of the meeting was devoted to a fluence of Bernal and Bragg was clearly paramount. Bernal saw these descriptions recreation of the events that led to the pep­ crucial. A typical contribution was as the first stage - their analysis should sin crystal's coming to Bernal's laboratory Bernal's letter to Dorothy Crowfoot sug­ lead to the principles which govern the at Camt ridge and to the crystallo­ gesting that the isomorphous replacement assembly and integration of molecules into graphic examination of pepsin and of Zn by Cd in the rhombohedral insulin higher organizations. Already, as Richard other protein crystals (including viruses) crystal could give phase information. Henderson emphasized, crystals of very that subsequently took place. The Bragg's role, more concentrated at large molecular assemblies such as the historical accounts were filled out, first on haemoglobin and later on nucleosome have been solved. Thus, as sometimes straightened out, particularly myoglobin, was quieter but his steady sym­ long as there are crystals, the future of the by John Philpot on the background of the pathy, persistence and deep crystallo­ field is assured and more than jUstifies the crystallization of pepsin (in explaining why graphic insight helped the early research great excitement produced by those first he went to Uppsala to grow the crystals he through disappointing years. Perutz re­ X-ray photographs of pepsin. 0 distinguished nicely between his real reason called Bragg's grasp of crystallographic for going there - a young lady - and his principles and illustrated it by the studies Guy Dodson is in the Department ojChemistry, excuse - to work on the ultracentrifuge) on how the shrinkage of protein crystal University oj York, York Y015DD, and Cyrus Chothia is at the Medical Research Council, and by Bill Pirie on early virus experiments. cellS could be used to define centric phases. Laboratory oj Molecular Biology, Cambridge The history of pepsin was completed by a In his review of virus research and the CB2 2QH, and the Department oj Chemistry, comment from Tom Blundell on the struc­ structure of water, both deep interests of University College oj London. ture of the pepsin molecule inferred from homologous enzymes. The impressions that emerged of Bernal's laboratory were of great activity and many abilities, and of Bernal's omni­ vorous scientific interests. The laboratory already had a famous reputation, par­ ticularly for the work on sterols, and it was this, or Dorothy Crowfoot's reputation, that apparently prompted Olen Millikan, a peripatetic scientist, to bring John Philpot's crystals to Cambridge. When the crystals arrived, Bernal im­ mediately examined them in a polarizing microscope (Dorothy Crowfoot was away with what proved to be an incipient rheu­ matic attack). He found them to be beauti­ fully formed, showing sharp birefringence while in the tube they had been grown in. Dorothy Hodgkin, John Philpot and Helen Megaw at the meeting. (Photograph by Claudio Villa.) © 1984 Nature Publishing Group