Seeing and Knowing in Structural Biology
2007; Elsevier BV; Volume: 282; Issue: 45 Linguagem: Inglês
10.1074/jbc.x700001200
ISSN1083-351X
Autores Tópico(s)Photosynthetic Processes and Mechanisms
ResumoChance often determines how a young person finds her calling. In 1949, I was majoring in Biology and Physics at Bryn Mawr College. I took a summer job (as usual) to earn money, this time waitressing at a resort hotel in the Pocono Mountains. We were given uniforms, including hairnets, and were told to prepare for training. The training turned out to be daily practice in serving the older, permanent waitresses, who did not tip and did not like Jews. After a few weeks, I was finally allowed to work in the main dining room. I served my first breakfast to a family of five, my hand shaking as I lowered their juice glasses to the table, and I received my first tip, which was negligible. Feeling hopeless, I telephoned a friend from Bryn Mawr who was spending the summer in Woods Hole, MA, where she worked in a place called the Marine Biological Laboratory (MBL). She urged me to join her there and promised to help me find a job for the rest of the summer. That job gave direction to my scientific life. I worked happily in the kitchen of the MBL, tyrannized by the beloved Miss Bell, who treated everyone alike. In my free time, I dissected squid axons (badly) for Otto Schmitt and attended all the lectures I could find. One of the lecturers was a brilliant English mathematician and crystallographer by the name of Dorothy Wrinch. She presented her views (later disproved) on the atomic structures of proteins, with strikingly beautiful slides. I understood little of what she said that summer, but what I saw in her pictures persuaded me to work on the structure of proteins. Among the scientists I met at the MBL was young Shinya Inoué, who was then a graduate student at Princeton. He was working on an early version of his now famous polarizing microscope. In a small, darkened room, he showed me a living, unstained egg from Chaetopterus (a marine worm) in polarized light. The spindle fibers were brilliantly clear, and one could detect the “fibrils” (now known to be microtubules) of which they were composed. He then put a Petri dish with the eggs on some crushed ice, and when I looked again, the spindle had disappeared. The mysterious fibrils had depolymerized at low temperature; and this effect was rapidly reversed upon raising the temperature. These discoveries of Shinya showed me that proteins have curious properties, which we, in the late 1940s, were only beginning to understand. There it was: I needed to see as much as I could of these marvelous materials. And I needed to comprehend what their images could reveal. Seeing and knowing about protein structures became the main goals of my professional life. I chose to go to graduate school at the Massachusetts Institute of Technology (MIT), where they had a biophysics program with a specialty called “Ultrastructure.” It was a joy to have started graduate work in 1950 when virtually nothing was known about the structure of biological molecules. By the time I graduated in 1954, the α-helix had been proposed by Pauling at Cal Tech, and the DNA double helix deciphered by Watson and Crick at Cambridge and Franklin at King's College, London. The fields that we now call structural biology and molecular biology were emerging (1Kendrew J.C. How molecular biology started.Sci. Amer. 1967; 216: 141-143Crossref Google Scholar). At MIT, under the mentorship of Richard Bear, a kind, modest, profoundly intelligent man and a pioneer in the x-ray diffraction of fibrous proteins, I tackled two problems for my graduate thesis. Both taught me the same lesson. One goal was to determine the structure of the fibrous protein collagen from its rather meager x-ray fiber diagram. Here, thanks to helical diffraction theory, developed by Cochran, Crick, and Vand in 1951 (2Cochran W. Crick F.H.C. Vand C. The structure of synthetic polypeptides. I. The transform of atoms on a helix.Acta Crystallogr. 1952; 5: 581-586Crossref Google Scholar), I knew that I was dealing with a helix and could define its parameters (3Cohen C. Bear R.S. Helical polypeptide chain configuration in collagen.J. Am. Chem. Soc. 1953; 75: 2783Crossref Scopus (26) Google Scholar). But I encountered ambiguities for the exact run of the polypeptide chains and could not resolve them by model building. (I might add that I had to go to the Harvard Botanical Library to examine the old papers on phyllotaxis (the study of the helical arrangement of leaves on plants) to categorize these ambiguities (4Cohen C. The Helical Configuration of the Polypeptide Chains in Collagen. 1954; (Ph.D. thesis, Massachusetts Institute of Technology)Google Scholar).) The covalent connectivity of the helices was discovered soon afterward by the insightful work of Ramachandran, followed by that of Rich and Crick. I also tackled the problem of interpreting the optical rotatory properties of proteins, including collagen, to try to deduce the sense of twist of the helices they contained (5Cohen C. Optical rotation and polypeptide chain configuration in proteins.Nature. 1955; 175: 129-132Crossref PubMed Scopus (10) Google Scholar, 6Cohen C. Optical rotation and helical polypeptide chain configuration in collagen and gelatin.J. Biophys. Biochem. Cytol. 1955; 1: 203-214Crossref PubMed Scopus (9) Google Scholar). The effects of a helical conformation on polypeptide chain conformation had not previously been examined. Here, the collaboration of a beloved classmate, Paul Gallop, was essential: he prepared the proteins, and I carried out the polarimetry studies. This effort was confounded at that time, however, because we did not know (and even Pauling did not know) whether α-helices were right- or left-handed. Or could they be both? So, early in my work, I encountered the ambiguity involved in interpreting images: seeing an image, or conceiving of one in the mind's eye, often leads to an enigma. In the solution of that enigma, “seeing” can become “knowing.” The word “enigma,” deriving from the Greek, means “to speak in riddles,” and the most famous riddle maker is, of course, the Delphic oracle from antiquity. Sitting on a tripod and speaking in a trance, she gave counsel in her so-called “maniacal chatter.” Rightly or wrongly, supplicants interpreted her riddles, which led them to their (sometimes dire) fate. Fortunately, today, in Structural Biology, our misinterpretations are usually not fatal, only embarrassing. In these reflections, I want to show, from my own early experiences and those of others, some of the strategies used in solving scientific riddles. When I was at MIT, my early lessons in interpreting images were reinforced by the studies of Hugh Huxley (from Cambridge University) and Jean Hanson (from King's College, London), who were there as postdoctoral fellows. Hugh was expert in the x-ray diffraction of muscle and was learning electron microscopy, as was Jean, who had previously worked in light microscopy. They collaborated in attempting to decipher the relationship between muscle structure and contraction. By 1942, Albert Szent-Györgyi, the Hungarian biochemist who had won a Nobel Prize for his discovery of vitamin C, discovered that two different proteins, myosin and actin, together are essential for contraction. Nevertheless, he and most others believed that there was only one continuous set of “actomyosin” filaments running through a myofibril in muscle and that their internal folding produced shortening. This notion was partly due to the fact that rather thick sections of muscle were being used in electron microscopy at the time. By 1953, however, at MIT, Hugh was obtaining very thin cross-sections of muscle from which he could distinguish two sets of filaments in the region of the so-called “A band” (7Huxley H.E. Electron microscope studies of the organization of the filaments in striated muscle.Biochim. Biophys. Acta. 1953; 12: 387-394Crossref PubMed Scopus (134) Google Scholar). I remember carefully following Hugh's and Jean's studies and seeing how they combined complex and ambiguous x-ray diffraction, light microscope, and electron microscope (EM) images to arrive at a rather radical new mechanism for muscle contraction: the sliding-filament theory (8Huxley H.E. Hanson J. Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation.Nature. 1954; 173: 973-976Crossref PubMed Scopus (914) Google Scholar, 9Hanson J. Huxley H.E. The structural basis of contraction in striated muscle.Symp. Soc. Exp. Biol. Fibrous Proteins and Their Biological Significance. 1955; 9: 228-264Google Scholar). In this theory, they pictured how the myosin molecules were, in effect, pulling the actin filaments into the thick filament lattice to produce shortening. A crucial point here is the nature of the evidence. Hugh's and Jean's ideas were accepted only after intense controversy and a variety of additional experimental results (10Huxley H.E. The double array of filaments in cross-striated muscle.J. Biophys. Biochem. Cytol. 1957; 3: 631-648Crossref PubMed Scopus (237) Google Scholar). I remember being struck by the painstaking approach they took; they carried out a number of experiments designed to obtain more and more conclusive evidence. But these same experiments could refute their theory if the results contradicted their hypothesis. In fact, trying to disprove one's own ideas is a common strategy for how to do good science! I should add that, not surprisingly, it was when I was in graduate school that I was converted to the muscle proteins. After I took my degree in 1954, I joined Jean Hanson as a postdoctoral fellow at King's College, and the problem I tackled there was the x-ray structure of actin filaments. A technician of Jean's prepared the material, using a method that had recently been developed by Albert Szent-Györgyi's young cousin, Andrew. (He and his wife, Eva, were then working in Albert's laboratory at the MBL in Woods Hole.) The material was very viscous, and I was able to pull highly oriented fibers by dipping forceps into the gel and spreading the tips. The x-ray work was carried out in a room close by, and I used Rosalind Franklin's microcamera to obtain rather good fiber diagrams. Franklin had been at King's until a year before I arrived. At that time, I knew almost nothing of the struggle over the structure of DNA that had so recently taken place between King's and Cambridge. Maurice Wilkins (called “Uncle” for his slow seriousness) was at King's, and he and I became quite friendly. In fact, the whole group of young scientists in the laboratory, including Pauline Cowan (later Harrison) and Stewart McGavin, was quite matey, and we often repaired to the nearby pub with Maurice on Friday afternoons. Little was said about Rosalind, but I did know that she had joined J. D. Bernal's group at Birkbeck College, so one day, I arranged to meet her. I duly climbed up to Rosalind's attic office/laboratory at 21 Torrington Square. (I recollect encountering her collaborator, Aaron Klug, on my way up.) She was in a very small room with a desk and chair; on a table behind her were many thin, upright capillary tubes. These contained the oriented samples of tobacco mosaic virus (TMV), her current focus. Rosalind was very gracious, and as she shook my hand, she said diffidently, “You know, I am not really a crystallographer.” I responded that I was not one either and that I just knew a bit about helical diffraction. She showed me her latest x-ray photographs of TMV, which were immensely detailed and very beautiful. And then I made one of my most reckless pronouncements: “You will never solve this structure,” I blurted out. Rather than saluting the perfection of her photographs, I was thinking of how hard it would be to disentangle overlapping Bessel functions. She was very quiet for a moment, and then we went down to a luncheon with several people, including the illustrious Bernal (called “Sage”), whom I met for the first time. He was told that my work was related to muscle contraction, so he proceeded to launch into some theoretical arguments on the subject. And then I behaved badly again; while he was holding forth, I turned to Rosalind to ask, in a whisper, how she could have left Paris for London and had she read Proust? No response. We then parted. She did not entertain me again. Some months later, I phoned and said, “Rosalind, I have some really nice fibers of actin, but I need a better camera.” She replied, “I am very sorry, Carolyn, but my camera is tied up all the time.” I understood her message. And within four years, she had disproved my prediction about TMV. Among her contributions, she first identified the location of the RNA (11Franklin R.E. Location of the ribonucleic acid in the tobacco mosaic virus particle.Nature. 1956; 177: 928Crossref Scopus (60) Google Scholar); then, in collaboration with Ken Holmes, she determined the symmetry of the helical arrangement of the subunits (12Franklin R.E. Holmes K.C. Tobacco Mosaic Virus: Application of the method of isomorphous replacement to the determination of the helical parameters and radial density distribution.Acta Crystallogr. 1958; 11: 213-220Crossref Google Scholar). After her death, Ken, Aaron Klug, and Don Caspar carried this work forward with great distinction. I spent only nine months of my fellowship in London, with side trips to Paris and Florence and some trips to Cambridge to consult with Crick about α-helices and to take some actin X-ray diagrams with Hugh. I then returned to MIT and Dick Bear's laboratory, happy to be home. Here, I finished my x-ray analysis of actin and was able to define the basic parameters of the structure, but was it helical or planar? (It was not until 1963, when negative staining for EM had been developed, that Jean Hanson and Jack Lowy finally visualized the topology of the actin helix.) I then began some biophysical studies with Andrew Szent-Györgyi, which I will soon describe. One day, most unexpectedly, I received a phone call from Rosalind; it had been about two years since our last interaction. She said, “Carolyn, I am visiting Andrew Szent-Györgyi in Woods Hole, and he has shown me some of your x-ray photographs. Why don't you come down so that we can talk?” I did not know that Rosalind was mortally ill at the time. (It was not until she returned to England that the cancer was diagnosed.) And, to my shame, I had not yet learned the lesson of forgiveness. So I did not make that simple trip, and I regret my decision to this day. When I began to collaborate with Andrew Szent-Györgyi at MIT, as Dick Bear had recommended, we began working on optical rotation and x-ray diffraction of α-fibrous proteins, especially those in muscle. Andrew proved to be a delightful partner; he used to bring the proteins with him from Woods Hole to Cambridge, where we carried out the studies. It then became a ritual for us to have dinner in Chinatown and, if possible, go to the Boston Garden to watch the tennis. We were unscrupulous at MIT and often “borrowed” pipettes from better-equipped laboratories (in particular, that of Jack Buchanan). I also used to commute to Woods Hole regularly for our consultations. As with Paul Gallop, our mutual foibles were an immense source of pleasure, and there was much laughing amid the scientific disasters. We were even mildly amused, at a large meeting, to find our work attributed to someone else and then to read our work rewritten, as if de novo, by an admiring colleague. Apparently, our work was of some interest. What we found, in fact, was that many of the fibrous proteins had domains composed of α-helical coiled coils as well as globular regions, and we could begin to relate their shapes to these features (13Cohen C. Szent-Györgyi A.G. Optical rotation and helical polypeptide chain configuration in alpha-proteins.J. Am. Chem. Soc. 1957; 79: 248Crossref Scopus (25) Google Scholar, 14Cohen C. Szent-Györgyi A.G. The alpha-class of fibrous muscle proteins.Proceedings of the 4th International Congress of Biochemistry, Vienna, September 1–6, 1958. 1960; VIII (Pergamon Press, London): 108-118Google Scholar). I should add that Andrew was also very patient when, after about a year of work, I abandoned him to act out an early fantasy of mine to go to medical school. Somehow, I convinced Boston University to give me a chance, and then I could work with Andrew only on weekends. Part of his patience was due to the fact that he, himself, had earned a medical degree in Hungary while managing to avoid most clinics. He was pessimistic about my future in this field, and rightly so. After less than a month, I came to my senses and realized (not too late, I hoped, for another student to take my place) that medical school was not for me. I never did regret this (mis)adventure, and over time, I became a scrupulous adviser to my own students about careers in medicine. I then resumed full-time laboratory work. And here I solved a small, delightful riddle and learned the important lesson that “Great men sometimes make great mistakes.” (I think that great women, in general, tend to be rather more judicious.) In this case, the great man was Linus Pauling, who certainly was one of my heroes. In 1951, he and Robert Corey had published a prodigious number of papers in Proceedings of the National Academy of Sciences of the United States of America on their truly monumental findings about the basic conformations of polypeptide chains (see especially Refs. 15Pauling L. Corey R.B. Branson H. The structure of proteins; two hydrogen-bonded helical configurations of the polypeptide chain.Proc. Natl. Acad. Sci. U. S. A. 1951; 37: 205-211Crossref PubMed Scopus (1958) Google Scholar and 16Pauling L. Corey R.B. The pleated sheet, a new layer configuration of polypeptide chains.Proc. Natl. Acad. Sci. U. S. A. 1951; 37: 251-256Crossref PubMed Scopus (413) Google Scholar; for a recent review, see Ref. 17Eisenberg D. The discovery of the alpha-helix and beta-sheet, the principal structural features of proteins.Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 11207-11210Crossref PubMed Scopus (185) Google Scholar). But in that same volume, they had published a long paper on “The Structure of Hair, Muscle, and Related Proteins” (18Pauling L. Corey R.B. The structure of hair, muscle, and related proteins.Proc. Natl. Acad. Sci. U. S. A. 1951; 37: 261-271Crossref PubMed Scopus (49) Google Scholar). Apparently, they believed that the report of Lotmar and Picken, in 1942, of “well crystallized” muscle (following Herzog and Jancke, in 1926) should be taken seriously. For several pages, they analyzed the reflections and speculated on the “muscle” structure. This paper was followed, however, by reports in Nature by Dick Bear and his student Cecily Cannan (19Bear R.S. Cannan C.M.M. The “Lotmar-Picken” X-ray diffraction diagram of muscle.Nature. 1951; 168: 684-687Crossref PubMed Scopus (0) Google Scholar) and by Hugh Huxley and John Kendrew (20Huxley H.E. Kendrew J.C. Extractability of the Lotmar-Picken material from dried muscle.Nature. 1952; 170: 882Crossref PubMed Scopus (4) Google Scholar) showing that a water-soluble, small, organic molecule was, in fact, the source of the “crystalline muscle” diagram. My contribution, in 1958, was simple: I noticed that most of the muscles that yielded this pattern were derived from mollusks. Seeking advice, I telephoned Betty Twarog, a physiologist friend of mine at the Harvard Biology Laboratories, to ask what substance might be found in high concentrations in molluscan muscle. She quickly named a few compounds, including taurine (an amino acid). I then soaked a washed frog sartorius muscle in a 5% solution of taurine, let it dry, and took the x-ray photograph. There was the Lotmar-Picken diagram! The note I soon published (21Cohen C. The Lotmar-Picken substance: Taurine.J. Biophys. Biochem. Cytol. 1958; 4: 489-490Crossref PubMed Scopus (1) Google Scholar) was just one paragraph in length and had just one figure. Would that more scientific experiments could be so simple and diverting! I first met Don Caspar before I finished my Ph.D. He visited me, rather unexpectedly, in Cambridge, MA, where I was then living. He was a close friend of a classmate of mine at Bryn Mawr, Ethel Stolzenberg (later Tessman). In 1945, I had met Ethel in New York City, when both of us were standing outside Barnard College, waiting for a scholarship interview. She was a Brooklyn original: her father was a revered Yiddish poet; her clarity and humor were incomparable. A year later, we met again at Bryn Mawr, where we were both majoring in Biology. Eventually, she developed the deluded notion that a biophysicist named Don Caspar was destined to become my life partner. When we finally met for the first time in Cambridge, Don was finishing his thesis at Yale, and I would soon be off to London. He said he was working on the small-angle equatorial diffraction of oriented TMV liquid crystals. This did not inspire my optimism, but, fortunately, I had laryngitis at the time and could not comment. While I worked at MIT, Don went to Cal Tech and then to Cambridge, UK, for a postdoctoral stint. After he returned to Yale, we began our collaboration in 1957. I had decided to try to solve the structure of the beautiful, highly hydrated tropomyosin crystals. These had been discovered, in the 1940s, by Kenneth Bailey at Cambridge, and I asked Andrew Szent-Györgyi to crystallize some tropomyosin for me. (I should note that some years earlier, when thinking about α-helices, Crick had carried out one of his rare experiments by trying to take an x-ray photograph of a tropomyosin crystal at Cambridge, but succeeded only in burning a large hole through the crystal.) I found that our x-ray equipment at MIT was too primitive to take precession photographs, so I telephoned Don, and he invited me to his laboratory at Yale. In November, I drove down from Boston to New Haven, where Don met me. We managed to get the fragile crystals into a capillary and then threw the windows open in the x-ray laboratory to cool the room while Don's postdoctoral fellow, Bob Langridge, set up the camera. That evening, Don put me up, fed me an excellent dinner, and gave me a copy of C. P. Snow's The Search to read. (This novel was based on Bernal's colorful career in the 1930s.) Next morning, we examined the photograph and were astonished: we saw two spikes of strong reflections, forming a dramatic cross. We knew we had an important result, but the photograph did not speak to us until some years later. After my post-doctoral years in the Bear laboratory at MIT, Dick was leaving to become a Dean at Iowa State University, and I was beginning to see that it was time to establish my own laboratory. I wanted to find a place where I would be free to carry out research and where no one would tell me what to do, a very privileged place, as I was well aware. (In those days, modest National Institutes of Health research grants were relatively easy to obtain.) I had become friends with Betty Geren (later Uzman), a post-MD student of Frank Schmitt at MIT. She had the great distinction of having discovered, by a series of electron microscopic studies, how myelin is formed in the nervous system. Betty had established an electron microscopy laboratory at the fledgling Children's Cancer Research Foundation in Boston (also called the Jimmy Fund), directed by the extraordinary pathologist Sidney Farber. Dr. Farber was a visionary about curing cancer, especially the kinds that afflict children. He was also a realist about money and politics at the Harvard Medical School, where he was one of the first Jews to become a professor. (He sometimes likened the “Quadrangle” at the Medical School to the Roman Arena.) After I set up a small x-ray laboratory for Betty, she convinced Dr. Farber to offer me some space of my own at the Jimmy Fund. (It was no accident, in those days, that many less-than-illustrious places recruited a number of first-rate women.) This seemed to me an excellent opportunity, and I was elated when Don Caspar agreed to abandon the tenure track at Yale to throw in his lot with me in a joint venture. That was the beginning of our happy commune: we shared the expenses of the equipment, and far better, we shared ideas. We established our laboratory about 1958, and the place began to flourish. Bob Langridge and Susan Lowey soon joined our commune. Susan, a physical chemist who had been working on myosin in John Edsall's laboratory at Harvard, started as a postdoctoral fellow with me. Together, we tried to envisage a structure for myosin by combining her expertise on the hydrodynamic behavior of myosin and the meromyosins with the information from wide-angle x-ray diffraction and optical rotatory dispersion that Andrew Szent-Györgyi and I had obtained (22Cohen C. Invited discussion.J. Polymer Sci. 1961; 49: 144-145Crossref Google Scholar). We conceived of a model for the molecule that had a “double-stranded α-helical core with globular mass(es) projecting from the rod” (23Lowey S. Cohen C. Studies on the structure of myosin.J. Mol. Biol. 1962; 4: 293-308Crossref PubMed Scopus (224) Google Scholar). Some five years later, Susan and Henry Slayter visualized myosin in the electron microscope and showed that there are, in fact, two globular heads to the molecule (24Slayter H.S. Lowey S. Substructure of the myosin molecule as visualized by electron microscopy.Proc. Natl. Acad. Sci. U. S. A. 1967; 58: 1611-1618Crossref PubMed Scopus (150) Google Scholar). This achievement was among the earlier of Susan's many notable contributions to studies on the biochemical dissection of myosin. Ken Holmes came over from England in the fall of 1959 to continue his work on TMV as a postdoctoral fellow with Don Caspar. Trained by Rosalind Franklin, Ken was expert with focusing monochromators, and he was soon helping me take diffraction photographs of Betty Twarog's favorite molluscan muscle, the anterior byssus retractor muscle of Mytilus edulis, which controls the byssus threads of mussels and enables them to cling to rocks. This so-called “catch muscle” had an ideal anatomy for physiologists to relate its mechanical and electrical properties. It also turned out to be an ideal specimen for displaying the diffraction pattern from α-helical coiled coils because of the large amount of the α-helical coiled-coil protein paramyosin in the core of the thick filaments. (This is another excellent example of how Nature always provides the “right” creature to answer any question a biologist might ask, analogous to the concept of “privileged materials” described by Louis Pasteur in connection with crystals of the tartrates that he exploited to discover the world of stereochemistry.) The intact cylindrical muscle could readily be pulled into a quartz capillary and yielded the first x-ray diagram of hydrated, well oriented α-helical coiled coils. This fiber diagram spoke plainly to Ken and me, and we were able to establish the two-chain structure of the molecules, which had been the subject of controversy until that time (25Cohen C. Holmes K.C. X-ray diffraction evidence for alphahelical coiled coils in native muscle.J. Mol. Biol. 1963; 6: 423-432Crossref PubMed Scopus (127) Google Scholar). But the patterns were not good enough to distinguish whether the chains were parallel or antiparallel, and we blundered by choosing the latter. It was only when Don and I finally made progress with tropomyosin (described below) that the answer became plain. In fact, it took until 1991 for the atomic details of an α-helical coiled coil to be established with the crystal structure of the leucine zipper GCN4 (26O'Shea E.K. Klemm J.D. Kim P.S. Alber T. X-ray structure of the GCN4 leucine zipper, a two-stranded, parallel coiled coil.Science. 1991; 254: 539-544Crossref PubMed Scopus (1279) Google Scholar). I should add that Ken (I trust because of our work together) switched his focus from virus structure to the muscle proteins, a field in which he has become a renowned expert. Another general point we made, based on the coiled-coil structure, was that the α-helix in any protein in aqueous solution acquires increased stabilization by intramolecular side chain interactions. In all proteins, hydrophobic bonding is a primary driving force for tertiary fold formation in aqueous solution. But the interactions of α-helices in globular proteins are complex: they do not display a simple, systematic scheme, as is found in coiled coils. Nevertheless, isolated regions of secondary structure, whatever their conformation, simply are not stable. When one does find such regions, they are often hinges or points of lability. This is a key message of the coiled coil. As I have described elsewhere (27Cohen C. Good times with Don Caspar.Biophys. J. 1998; 74: 532-533Abstract Full Text Full Text PDF PubMed Google Scholar), Don Caspar is a great explainer. He and Aaron Klug had been trying to explain the construction of icosahedral viruses to each other for a number of years. Following Rosalind Franklin's death in the spring of 1958, they worked together at the Medical Research Council (MRC) in Cambridge, UK, to write the paper on virus structures that she was to have presented at a symposium that winter. Later, and in part inspired by the design of geodesic domes by the architect Buckminster Fuller (another explainer), they made a major breakthrough. They accounted, with great precision, for the hitherto puzzling numerology found in the so-called “morphological” units that were seen, by EM, clustered on the surface of icosahedral viruses. They formulated, as well, the concept of self-assembly of biological structures (28Caspar D.L.D. Klug A. Physical principles in the construction of regular viruses.Cold Spring Harbor Symp. Quant. Biol. 1962; 27: 1-24Crossref PubMed Scopus (1892) Google Scholar) by showing how the design of the assembly is built into the bonding pattern of the subuni
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