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fractionation of liver homogenates. The TIMELINE emphasis was on the quantitative monitoring of the distribution of the chemical con- stituents of the cell, rather than organelle : charismatic purity5–8. Trial and error must have been the norm, and cold-room stamina a prerequisite, virtuoso of but this early period ultimately established the procedures that allowed organelles to remain intact without agglutination or lysis. Many Alan M. Tartakoff obstacles confronted these investigators, including a “…biochemical Zeitgeist that par- George Palade has created, shared and on the state of knowledge at that time in his ticles were a nuisance and stood in the way passed on a multidisciplinary view of the Nobel lecture,“…biologists [had been] in the of purification of … enzymes.”9 Whereas functional organization, biogenesis and same situation as astronomers and astro- was developing rapidly, the dynamics of organelles. His open- physicists, who were permitted to see the understanding of the compartmentalization mindedness and tenacity, along with his objects of their interest, but not to touch of subcellular activities and the significance of rigour and sense of intellectual elegance, them; the cell was as distant from us as the organelles was still in its infancy. have been remarkable. This focus on the stars and galaxies were from them. More dra- Claude returned to his native Belgium in logic of organelles defined a crucial turning matic and frustrating was that we knew that 1949, but not before he and his colleagues point in biomedical science. The following the instrument at our disposal, the micro- had systematized the use of differential sedi- article sketches Palade’s research, as part scope … had … reached, irremediably, the mentation to isolate a comprehensive set of of a larger community that flourished after theoretical limits of its resolving power.”4 fractions from tissue homogenates using the Second World War. Claude had begun his investigations of hypertonic sucrose (in place of saline or organelle isolation by sedimenting intact cells, water) as a homogenization medium. These George Emil Palade (FIG.1) was born into an the organelles of which became beautifully fractions comprised heavy particles, a large- academic family in Moldavia, Romania. He stratified, without the rupture of the plasma particle fraction that was enriched in mito- received his medical training at the School of membrane (see TIMELINE). As a result of his chondria and , a light fraction that Medicine in Bucharest and carried out his efforts to isolate the Rous sarcoma virus, he was enriched in microsomes — membrane first research as part of a doctoral thesis on then — in conjunction with the Canadian- vesicles which were so small that they could the kidneys of dolphins, which were obtained born developmental biologist Keith Porter not be seen with the microscopes that were with the help of Black-Sea fishermen. He then (FIG. 1), George Hogeboom and Walter available at the time — and a soluble fraction. served on the faculty of the Institute of Schneider — undertook the development of These fractions were studied both biochemi- at the University of Bucharest until procedures for the systematic subcellular cally and by using the electron microscope — having survived the Nazi threat — in 1946, at the age of 34, he moved with his wife and two small children to New York City 1-3.

New methods and approaches Palade spent his first few months in the United States with Robert Chambers at New York University, where he studied cellular membranes. After seeing the first of Albert Claude’s electron micrographs, however, Palade joined Claude (FIG. 1) — initially as a volunteer — in the Department of Figure 1 | Portraits. From left to right: George Palade (reproduced with permission from REF. 1 © The and Bacteriology at the Rockefeller Institute, Press), Albert Claude (reproduced with permission from REF. 21 © (2002) The where the young doctor Claude had arrived Rockefeller University Press), Keith Porter (photograph courtesy of Blair Bowers) and from Belgium in 1929. Claude commented (© The Nobel Foundation).

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Timeline | Microscopy and subcellular fractionation

×270 visualization Magnifying of bacteria (van Acceptance of the Abbé’s treatise Apochromatic Description of Successful glasses ×5–10 Leeuwenhoek) cell theory on optics objectives ergastoplasm (Garnier) osmium fixation

~1500 ~1650 1683 1823 ~1850 1869 1876 1878 1886 1896 1897–1898 1898 1927

Magnifying glasses Achromatic lenses Miescher’s attempts Oil immersion First edition of The Cell Description of the ×30 visualization of (resolution: 1 micron) at isolation of nuclei lenses in Development and Golgi complex (Golgi) plant cell walls in Heredity24 (Wilson) cork (Hooke)

(EM), which scanned the stratified particulate 1953,“…what we were doing was trying to of the Rockefeller Institute.”11 “The new field contents of every pellet from the bottom to separate populations that … might at best be had virtually no tradition; everybody working the top10. And yet, as Christian de Duve (FIG. 1) only partly separable from each other. In in it came from some other province in nat- pointed out, before the introduction of den- addition, we were using a poorly discriminat- ural sciences … Added to all this excitement sity gradients and swinging bucket rotors in ing method for this purpose.”8 was a pervading free spirit — often irreverent, A few years earlier, in the period but always helpful, because it acted as an anti- 1945–1947, Porter, Claude, Ernest Fullam and dote against imagined grandeur. Keith Porter Edward Pickels had examined intact thinly was responsible for a good part of that spread cells using the earliest of the EMs, and spirit…”11. When Porter later left Rockefeller had already distinguished a lace-like structure University for Harvard University, and as a that later became known as the endoplasmic fond reminder of his contributions, the reticulum (ER)11,12. Some of the remaining Palade laboratory for many years displayed a technical hurdles were resolved by the intro- prominent photo of Porter with the caption duction of buffered osmium tetroxide for fixa- ‘Our Father Who Art at Harvard.’ tion (by Palade, in 1952), plastic (such as methacrylate) as an embedding medium (by Vesicular transport Newman, Borysko and Swerdlow, in 1949), In 1953, Palade described a ‘small particulate glass knives (by Latta and Hartmann, in 1950) component of the cytoplasm’ — which later and ingenious microtomes that produced thin became known as ribosomes. By 1956, the sections (by Joseph Blum and Porter), which painstaking microscopic and biochemical became commercially available in 1953. characterization of microsomal fractions was Our present knowledge of the fundamen- published by Palade and , who tal properties of the cell is, largely, a function had joined the group in 1955. This work veri- of the early solutions to these methodological fied the hypothesis that the main component problems. Considering the novelty of the of the fraction was derived from fragmented procedures that were developed and the ER13. With the procedures established for sepa- refinements that were made over the years rating fractions enriched in microsomes from that followed, it is a great tribute to these those enriched in granules, Siekevitz investigators that the quality of their EM and Palade then initiated biochemical investi- images and subcellular fractionation in many gations of macromolecular transport along the cases match present standards. Figure 2 | Hepatocyte membrane differentiation. This electron micrograph was published as part of a Summing up this period — when many of study of membrane induction during hepatocyte the leading cell biologists of the future passed differentiation. Note the rough-surfaced cisternae of through the laboratory5,7,11 — Palade wrote “These concepts have the (ER), which include some that “After so many years, it is difficult to become so central to our areas without attached ribosomes (long arrows). recapture in words the atmosphere of intense Also note the relative scarcity of ribosomes in activity, remarkable achievements, great thinking that the grazing sections of the ER (short arrows). A few cisternal granules (cg) are present within the ER. excitement, and unlimited optimism that pre- intellectual advance that Microbodies (mb) and mitochondria (m) are also vailed in the laboratory, which otherwise they represent has gone included. Reproduced with permission from REF.22 looked like an unattractive dungeon sunk in © (2002) The Rockefeller University Press. the third basement of one of the old buildings almost unrecognized.”

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Production of commercial Introduction of Introduction of Isolation of microsomes phase microscopes that methacrylate for buffered Satisfactory routine using centrifuges capable were based on Zernike’s embedment (Newman, osmium fixative plastic resins for of 18,000 x g (Claude) model (Nobel prize 1953) Borysko and Swerdlow) (Palade) embedment (Luft)

1934 1938 1939 1941 1945 1949 1950 1952 1953 1961 1963

Attempts at isolation of Production of Examination of thinly- Glass knives for thin Commercial swinging bucket Introduction of mitochondria (Bensley) commercial electron spread cells in the electron sectioning (Latta rotors for ultracentrifugation glutaraldehyde microscopes (Siemens) microscope (Porter, and Hartmann) for EM fixation that were based on Claude and Fullam) Commercial Porter-Blum and (Sabatini) Ruska’s model (Nobel LKB ultramicrotomes (Porter, prize 1986) Blum and Sjöstrand)

EM visualization of ribosomes (Palade)

secretory pathway, which came to be the cen- Vectorial transport tral theme of the laboratory. In these classic The identification of the rough ER as the site ‘pulse-chase’ studies (which were first con- of synthesis of proteins that enter the secre- ducted by intravenous labelling with radioac- tory pathway drew attention to the distinct tive amino acids), their selection of the biosynthetic repertoires of ‘free’ versus exocrine pancreas as an experimental model ‘attached’ polysomes. Moreover, these obser- was key. This is because — as was proved vations had profound topological implica- much later — the acinar cells are so highly tions, and clearly showed that the lumen (or specialized for the synthesis of secretory cisternal space) of the ER should be equated proteins that almost all of the label was incor- with the extracellular space (FIG. 2).The porated into them. For the parallel EM- resulting fascination with the organization autoradiographic aspect of this research (for of the rough ER (and the rough microsomes which Lucien Caro was first responsible), the derived from it) led Palade and Siekevitz to light-microscopic methods of Charles develop methods for its disassembly — the Leblond needed further development, as removal of ribosomes and extraction of its there had been no previous demonstration content. David Sabatini quickly emerged as that thin sections that were exposed to photo- the central figure in these studies, which graphic emulsion could also be used for EM began as part of his doctoral thesis. Other visualization of ultrastructure. key figures were Mark Adelman, Günter The role of the Golgi complex as an Blobel, Nica Borgese, Gert Kreibich, James obligatory intermediate along the secretory Lake, Takashi Morimoto, Colvin Redman pathway, and the evidence for a post-Golgi and Yukata Tashiro. Their observations intermediate (the condensing vacuoles), came indicated that nascent polypeptide chains into focus in the studies initiated by James make a major contribution to the affinity of Jamieson during his graduate work. ribosomes for membranes, and allowed the Moreover, between 1967 and 1971, Jamieson crucial first demonstration of vectorial then developed what at first seemed a risky transport — termination of these pre-exist- approach for a tissue so full of destructive ing chains resulted in their release into the Figure 3 | Microdomains of the capillary enzymes: a system of tissue slices and, later, a lumen of the microsome. These seminal endothelium. These exquisite electron system of pancreatic lobules14,15. These studies findings clearly anticipated the later analysis micrographs were published as part of a series — as well as related studies of the parotid of the membrane translocation of individual of studies of vesicular transport across capillary endothelia. They illustrate pancreatic capillaries gland, the kinetics of transport, the composi- proteins, synthesized in vitro, into the ER 1–2 min after in situ perfusion of cationized tion of the membranes of the successive com- lumen, along with the cleavage of their signal ferritin. Binding of the electron-dense tracer to partments, and the proof of the equivalency peptides, as summarized in Günter Blobel’s the fenestral diaphragms (f) is intense, whereas of the pancreatic secretion and the content of Nobel lecture16. the membranes and diaphragms of secretion granules — were contributed by This fundamental information — on the plasmalemmal vesicles (v) are unlabelled. Note successive students and post-doctoral fellows itinerary of transport along the secretory the intense labelling of the coated pit (cp). Other areas of the plasmalemma show regional during that time: Abraham Amsterdam, pathway and the vital topological leap across variation (arrows and arrowheads). e, David Castle, Lewis Greene, Horst Kern, the ER membrane — set the stage for the endothelium; pc, pericyte. Reproduced with Jacopo Meldolesi, Hubert Reggio, George molecular identification of the functional permission from REF. 23 © (2002) The Scheele, Alan Tartakoff and Daniel Zagury. components that mediate these events. Rockefeller University Press.

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of the day and night. Distinguished visitors and ambitious young scientists arrived for a day, a month, or for a sabbatical semester, which made for a dynamic and very interna- tional environment. And George, along with his teaching commitments, travels and numerous meetings, seemed always to be available, willing to take one’s data even more seriously than one might have taken them oneself. He was intensely interested in all, and ultimately dedicated his Nobel lecture to the colleagues with whom he had worked14.And the commitment was mutual — once, when returning from a trip, he found his office dec- orated with balloons as a welcome that was orchestrated by Louise Evans. These were the days when he was the master electron microscopist for many pro- jects, when books were dedicated to him, and when a marvelously positive mood pre- vailed, with emphasis on thorough under- standing and documentation, rather than Figure 4 | Representatives of the Palade ‘ensemble’. The Palades have spent many summers in rapid publication. If he was caught up with Aspen, Colorado, attending the Aspen music festival. This photo shows George and some the tensions and competitive spirit that representatives of his ‘ensemble’ at a meeting in Kannami Tagata Shizuoka, Japan, in 1984. A pensive plague too many modern scientists, he George Palade is seated at the front listening to a performance on the koto. Later in the evening, he gave an informal exposition on European demographics and history. On other occasions, as described by spared his group the evidence of these pre- David Sabatini2, he shared his impassioned knowledge of medieval and Renaissance culture. In the occupations. background, from left to right, are Yutaka Tashiro, Alan Tartakoff, William Sly, Fred Maxfield and Bernhard An anecdote of Pietro De Camilli’s cap- Dobberstein. Photograph courtesy of A.M.T. tures some of George’s unusual qualities: as part of a long-standing tradition of exchanges, George periodically visited the Varieties of macromolecular transport The cell biology lab in the 1960s Istituto di Farmacologia in Milan. Before his The period of 1960–1975 was also a time of As the group grew, matured and filled the arrival — quite unlike the preparations important productivity in many related fifth floor of ‘South lab’ at Rockefeller accorded to mortal visitors — the entire areas. This included investigations of the University — where the halls were decorated Institute was scoured and each investigator induction and turnover of the membrane with superb electron micrographs — it came would carefully prepare a dossier of docu- proteins of the ER (Walter Bock, Gustav to exemplify a style of laboratory organization ments to discuss. When it came to the time Dallner, Lars Ernster, Andrea Leskes, Tsuneo that is unfortunately all too rare. The subject for Pietro to describe his annual progress, Omura and Christopher Widnell), as well as matter was diverse, yet the fascination with however, George assured him that there was studies of the biogenesis of mem- the cell biology of organelles and macromole- no need to re-examine the work they had branes of Chlamydomonas reinhardtii (Lilly cular transport gave it coherence. Perhaps discussed the preceding year — he still had it Bourguignon, Nam-Hai Chua, Benita most importantly, the younger members of in mind. dePetrocellis, Ken Hoober, Itshak Ohad, Ruth the faculty were independent and published Sager and Seth Schor). Günter Blobel, primarily on their own, yet they kept their Research after the Prize Rachele Maggio and Ariane Monneron puri- laboratories on the same floor. The result was The Nobel Prize is often awarded to leaders fied and characterized nuclei, and — follow- a community in which there was a real sense who have solved one conceptual puzzle of ing on from the seminal studies of James of solidarity. These ties also included individ- large importance or have provided an Morré and Alex Novikoff — John Bergeron, uals who were not situated on the same floor, invaluable practical resource. It has now John Ehrenreich and Kathryn Howell learned but who were nevertheless part of the Palade become clear that the contributions of to subfractionate the Golgi complex. Among ‘ensemble’,in part thanks to the all-important Claude, de Duve and Palade, who shared the the most exciting investigations carried out at Rockefeller lunch room. The constant senior Prize for Physiology or Medicine in 1974, this time were the exquisite EM studies of members of the group by the late 1960s included such conceptual and methodologi- junctional complexes and endocytosis in included Farquhar (Mrs Palade as of 1970) as cal conquests. But the essence of their con- epithelial cells (Marilyn Farquhar and Fritz well as Siekevitz, Jamieson, Sabatini and tribution was of a different, larger-scale Miller), experimental studies of vesicular David Luck. Indeed, from this time onwards it nature — they showed the importance of transport of electron-dense cargoes across became increasingly clear that enthusiasm focusing on the logic of biological organiza- endothelia (Romaine Bruns, Francesco and joy for science, which were exemplified by tion at the level of organelles, and convinced Clementi, Guido Majno, Maya and Nicolae George and Marilyn, were at the heart of this many of their peers that understanding Simionescu, Ernest Weibel and Sandy vigorous community. would lie at the intersection of the coordi- Wissig) and the first studies of the red-blood- Long hours were the norm. Sucrose gradi- nated alignment of several experimental cell membrane (Vincent Marchesi). ents were drained and EMs used at all hours approaches.

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Mingjie Jin, Janet Larkin, Lucian Saucan and Box 1 | Palade’s views on science in the United States — present and future Elizabeth Sztul), both EM and biochemical In tracing the phenomenal success of biomedical research, Palade singles out Vannevar Bush studies of endothelial transport (FIG. 3) as the visionary who motivated the US government to establish the National Science (Patrick Bankston, William Carley, Reinhard Foundation (1948) and the NIH (1950), which function in conjunction with ‘research- Horvat, Kaori Ihida, Bruce Jacobson, intensive universities’ that are distributed across the country and derive support from peer Anthony Milici, Ruediger Peters, Glyn Porter, review. The potentially fertile relationship with the private sector, in his estimation, has been Dan and Sanda Predescu, Vincent less successful, because “…too few mechanisms have emerged for encouraging their ‘far- Quagliarello, Gregory Roberts, Jan Schnitzer, 19 sightedness and generosity.’” . Radu-Virgil Stan and Hans Stukenbrok), as Despite the active encouragement of ‘bench-to-bedside’ research by others, Palade remained well as further methodological developments (in 1978) sceptical about what could be done.“There are no longer Maecenas, only government (John Gershoni, Akio Ito, Elaine Merisko and agencies and a few foundations. The scientists in need of support (the partial present-day Iwao Ohtsuki). equivalents of the philosophers and writers of times past) may have gained in dignity, but they have to pay for it in increased accountability … There is nothing wrong with accountability … Work away from the bench provided that its currency…[is] well chosen. The present currency — published papers — is Outside the laboratory (FIG. 4; BOX 1), Palade inherently inflationary. Unless checked, it may lead to the devaluation of the scientific literature. And the current short terms within which results are expected are profoundly unrealistic, as we assumed many responsibilities on National are finding now by reassessing the results of our recent campaigns against cancer and Institutes of Health (NIH) committees, par- cardiovascular disease … The scientists must [show] that it is … basic [knowledge which] is ticipated actively in establishing the Journal needed … It is in this context that the paradoxes of our present condition have developed: a of Cell Biology in 1955 (originally the Journal 18 remarkable, historically unique record that is not properly recognized; a successful effort that of Biochemical and Biophysical Cytology) as may come to a premature halt for lack of adequate support; a sophisticated society that fails to well as the Annual Reviews of Cell Biology in understand clearly where we stand in our efforts to control major diseases; and the insistence on 1985 (now the Annual Reviews of Cell and storming targets with a lot of money and a modicum of uncertain leads.”20 Developmental Biology) and organized the first faculty senate at Rockefeller University. He is credited with having introduced poster sessions at the annual meetings of the The functional, temporal and spatial conti- vesicle to disperse into the membrane matrix American Society for Cell Biology, of which nuity of membranes. The studies of of a terminus, and … recapture these compo- he was President in 1976. Moreover, he has Siekevitz and Palade (and others) showed nents before non-random removal from that helped make it possible for many talented that membrane proteins of the secretory terminus is effected…”17. researchers from his native Romania to pur- pathway turn over only very slowly. This sue their training in the United States, and realization raised fundamental questions Organelle dynamics and macromolecular he has worked to establish an institute in about the mechanism by which transport of transport. During this period, the laboratory Bucharest that is directed by his close rapidly synthesized proteins is accom- studied many aspects of organelle dynamics friends and colleagues, Maya and (the late) plished. In particular, the numerous compo- and macromolecular transport: the biogene- Nicolae Simionescu. sitional and functional differences between sis of red-blood-cell-surface proteins In 1973, after 28 years at Rockefeller successive compartments made it clear that (Elizabeth Dolci, Andreas Sarris, Jeffrey University, George and Marilyn moved to most membrane components certainly did Ulmer and Bridget Wilson), the secretory , where George became not ‘flow’ along with their cargoes. Palade pathway of acinar cells of the pancreas (Linda Chairman of the Section of Cell Biology and inferred that “A eukaryotic animal cell … Hendricks, Elaine Merisko and Jaakko served as Special Advisor to the Dean of the appears to operate a number of discontinu- Saraste), transcytosis (Michael Caplan, School of Medicine. In 1990, they moved to ous circulatory systems … the receiving compartment [acting] as an efficient sink so that backflow or diffusion up the pathway is prevented...”17. These concepts have become so central to our thinking that the intellec- tual advance that they represent has gone almost unrecognized. He then went much further by laying out a basic model to account for the specificity of interactions between the membrane carriers that mediate transport, and in which he pos- tulated the existence of successive mem- brane-bound signals and receptors17. He also realized that the tendency of proteins and lipids to diffuse in the plane of membranes (membrane fluidity) might randomize the composition of sequential compartments, Figure 5 | Photographs from the Palade Symposium in (March 25, 1999). Top row (from left and therefore he postulated the existence of to right): Maya Simionescu, George Palade, Marilyn Farquhar, James Jamieson and Günter Blobel. “…stabilizing infrastructures [which] allow Bottom row (from left to right): Pietro De Camilli, Karl Pfenninger, Nica Borgese, Louise Evans, Dorothy the membrane components of an incoming Bainton, Kathryn Howell, and Bernie Gilula. Photographs courtesy of Marilyn Farquhar.

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 3 | NOVEMBER 2002 | 875 PERSPECTIVES the University of California in San Diego 19. Palade, G. E. The American academia and the private 605–613 (1981). sector. Kos 95–96, 23–28 (1993). 24. Wilson, E. B. The Cell in Development and Heredity where, in addition to maintaining their 20. Palade, G. E. in Transport of Macromolecules in (Macmillan, New York, 1896). research programmes, George became Dean Cellular Systems (ed. Silverstein, S. C.) 517–522 (Dahlem Konferenzen, Berlin, 1978). of Scientific Affairs at the School of 21. Hanson, E. Achievements: A Century of Science for the Online links Medicine. Benefit of Humankind. 1901–2001 (Rockefeller Univ. Press, New York, 2000). FURTHER INFORMATION 22. Dallner, G., Siekevitz, P. & Palade, G. Biogenesis of Alan M. Tartakoff’s lab: Concluding remarks endoplasmic reticulum membranes. J. Cell Biol. 30, http://www.cwru.edu/med/pathology/fac/tartakoff.htm At a recent one-day symposium that was held 73–96 (1966). George E. Palade’s Nobel lecture: 23. Simionescu, N., Simionescu, M. & Palade, G. http://www.nobel.se/medicine/laureates/1974/palade- in his honour in La Jolla (FIG. 5),George Differentiated microdomains on the luminal lecture.pdf sounded like the son of the professor of phi- surface of the capillary endothelium. J. Cell Biol. 90, Access to this interactive links box is free online. losophy that he was. He reminded us that, although we have come far since the 1940s, we still have very far to travel. Our collective intellectual itinerary cannot be charted in OPINION advance, and will be enriched beyond all expectation as more approaches and experi- mental systems become part of our arma- mentarium. Those who identify with The significance of molecular slips in George’s enduring commitment to an inte- grative view of the dynamics of organelles transport systems were especially moved when he showed a list of former colleagues, and said that it “…reads like a poem.”Regardless of whether one has Nathan Nelson, Ayelet Sacher and Hannah Nelson worked directly with him or not, it has always been a great privilege to be part of George’s The advantage of precision in biological such as nucleic-acid replication, photosynthe- ensemble. processes is obvious; however, in many sis and respiration, but also modulates the Alan M. Tartakoff is at the Case Western Reserve cases, deviations from the faithful function of transport systems. This slippage in University School of Medicine, mechanisms occur. Here, we discuss how the driving force and/or in the transport path- Cleveland, Ohio 44106, USA. in-built operating imperfections in transport way might provide a safety valve through e-mail: [email protected] systems can actually benefit a cell. which an excess driving force or substrate is doi:10.1038/nrm953 dissipated. We propose that slips are funda- Without the genetic property of slight blun- mental for life processes, as they fulfil the cell’s 1. Porter, K. An informal tribute to George E. Palade. J. Cell Biol. 97, D4–D7 (1983). dering, the evolution of advanced life forms need to deal with stressful situations that are 2. Sabatini, D. George E. Palade: charting the secretory would be impossible — random mutations, created by transitory extreme conditions, pathway. Trends Cell Biol. 9, 413–417 (1999). as well as imperfect DNA replication and which might otherwise interfere with the 3. ASCB Profile of George Palade. ASCB Newslett. 1 23, 8–10 (2000). repair, are essential to life . Indeed, imperfec- proper function of a cell and even jeopardize 4. Claude, A. The coming of age of the cell. Science tions are necessary for the adaptation and its existence. To illustrate this point, we discuss 189, 433–435 (1975). 5. Palade, G. E. Albert Claude and the beginnings of smoothness of operation in many biological proton ATPases and the ion- or substrate- biological electron microscopy. J. Cell Biol. 50, 5D–19D processes2–4. translocation pathways of ion-motive trans- (1971). 6. Claude, A. Studies on cells: morphology, chemical Certain biochemical processes function by porters as examples of proteins that might constitution, and distribution of biochemical functions. coupling extremely precise reactions with oth- have adapted such in-built molecular slips. Harvey Lect. 43, 121–164 (1948). 7. Porter, K. R. George Hall Hogeboom (1913–1956). ers that enjoy a high degree of freedom. Take J. Biophys. Biochem. Cytol. 2, ix –xvi (1956). photosynthesis, for example. The structure of Ion-motive ATPases 8. de Duve, C. Exploring cells with a centrifuge. Science the reaction centres that harvest light and con- Evolutionarily, ion-motive ATPases can be 189, 186–194 (1975). 9. Lehninger, A. L. The ; Molecular Basis of vert its energy into electrochemical energy is grouped into two distinct families: the P- Structure and Function (W. A. Benjamin, New York, 1964). very rigid, and this allows only a small degree ATPases, which operate by means of a 10. Ernster, L. & Schatz, G. E. Mitochondria: a historical 5 review. J. Cell Biol. 91, 227s–255s (1981). of freedom . On the other hand, the electro- phospho-enzyme intermediate, and the F- 11. Palade, G. E. Keith Roberts Porter and the ∆µ chemical gradient of protons ( H+), which is and V-ATPases, which operate by means of development of contemporary cell biology. J. Cell Biol. 8 75, D3–D18 (1977). formed across the thylakoid membrane by the mechanochemical movements .However, 12. Pease, D. C. & Porter, K. R. Electron microscopy. reaction centres, is prone to losses through all of these ATP-dependent ion pumps cou- J. Cell Biol. 91, 287s–292s (1981). 13. Siekevitz, P. & Zamecnik, P. Ribosomes and protein ‘leaks’ and ‘slips’ (BOX 1). Therefore, the ratio of ple a scalar reaction of ATP hydrolysis (or synthesis. J. Cell Biol. 91, 53s–65s (1981). ATP synthesized to electrons transported does 14. Palade, G. E. Intracellular aspects of the process of protein secretion. Science 189, 347–358 (1975). not give a fixed stoichiometry, and the combi- 15. Scheele, G. Pancreatic lobules in the in vitro study of nation of a rigid photochemical reaction with pancreatic acinar cell function. Methods Enzymol. 98, biochemical reactions that are prone to leaks 17–28 (1983). “…imperfections are 16. Blobel, G. . Chembiochem. 1, 86–102 and slips has evolved to optimize photosyn- (2000). thesis. Similar arrangements of coupling and necessary for the adaptation 17. Palade, G. E. in Symposium on Membrane Recycling (eds Evered, D. & Collins, G.) 1–14 (Ciba Foundation slippage mechanisms operate in the respira- and smoothness of Symp. 92, Pitman Press, Bath, UK, 1982). tory chain6,7. 18. Porter, K. R. & Bennett, H. Stanley. Recollections on operation in many the beginning of the . J. Cell Biol. Here, we discuss the possibility that a slip 91 (Suppl.), vii–ix (1981). mechanism not only exists in complex systems, biological processes.”

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