The view from inside the Golgi complex
One of cell biology's leading lights, newly arrived from London, takes a bit of the mystery out of cell division.
The inside of a cell is a highly organized place, where structure and order allow its components to carry out their life-sustaining work. Nowhere is this more apparent than within the Golgi apparatus, the organelle that textbook authors variously describe as part factory, part shipping station, part switchboard and, in the words of one, “the Grand Central Station of all membrane traffic.”
Imagine a stack of pancake- or pita-shaped disks through which many cell products must pass on their way to their final destinations. It sorts and modifies proteins, fats and sugars and manufactures glycolipids and sphingomyelin, molecules that help protect the membrane that surrounds the cell. The Golgi decides whether a given protein will leave the cell or be delivered to the plasma membrane or another destination within the cell. Not only does it decide what goes where, but also how the parcels should be packaged for transport and how to apply the molecular tags that label them for proper delivery. It’s an extraordinarily complex task that scientists have painstakingly worked out during the century since Italian pathologist Camillo Golgi first identified the structure that bears his name.
Several years ago, Graham Warren, Ph.D., posed a new question about this remarkable organelle: When an ordinary cell divides, how does it provide the resulting daughter cells with roughly equal portions of Golgi to start them on their way in life? Over the last decade, scientists led by Warren have pieced together the precise choreography that allows the Golgi complex to disperse into roughly 10,000 tiny vesicles in a scant 10 minutes and reconstitute itself as two new structures in the cell’s progeny.
His research has shed light upon many other areas of cell biology and provided fundamental insight into how a cell distributes its organelles during cell division, with implications for better understanding, and perhaps treating, human disease, especially cancers, where cell division has run amok.
Warren brought this work, as well as his reputation as one of the world’s leading authorities in membrane transport, to Yale this summer, joining the Department of Cell Biology from his former position as principal scientist at the Imperial Cancer Research Fund (ICRF) in London. A Cambridge-trained biochemist, Warren, 51, spent eight years as a group leader at the European Molecular Biology Laboratory in Heidelberg, then chaired the biochemistry department at the University of Dundee in Scotland before joining ICRF in 1988. In June, he and his wife, Philippa, crossed the Atlantic and resettled in New Haven, leaving their four grown daughters behind in England. “A change of venue can be quite good for a scientist,” he said affably in his fourth-floor office overlooking Cedar Street in September, “by opening new lines of thought and creativity.”
His Yale colleagues will reap equal benefit, according to department chair Pietro De Camilli, M.D. “Graham is an intellectual force, and we expect him to be a catalyst of intellectual energy in the department,” De Camilli said. “We think of him as a very rigorous presence and a provocative colleague who will challenge and stimulate his fellow faculty.”
Colleagues say Warren was an excellent choice for the department founded by Nobelist George Palade, Ph.D., in 1974 because he links traditional approaches in cell biology to cutting-edge ideas on how to examine cells, their interior structures and their functions. “He was one of the first to apply molecular techniques to cell biology, and as such he fits in extremely well with the people who are already here,” said fellow professor Ira Mellman, Ph.D., who added that by combining biochemistry, structural biology and cell biology, Warren brings “an intellectual and methodological perspective which is fresh, new, and exciting.”
“Graham’s work,” said Mellman, “graphically demonstrates that the most successful biomedical scientists in the years to come will be those who can bridge and combine such diverse areas of expertise.”
What Warren has done in his own work is to lay out a blueprint for the method by which the Golgi divides itself. He says it is an important process that biology has largely overlooked in its preoccupation with understanding the sorting and distribution of DNA within the nucleus during mitosis. “The other organelles in the cell must have the means of assuring that they are partitioned equally in the cell,” he said. “It’s just not as obvious as when the chromosomes all get in line.”
Sketching on a whiteboard in his office, he leads a visitor through the process by which the Golgi normally moves proteins from unit to unit by encapsulating them in small transport vesicles bound either for the cell surface, other organelles, or the next pita-shaped cisterna in the Golgi stack. He says it was this particular step, the movement of the transport vesicles between individual cisternae, that suggested a mechanism by which the large-scale fragmentation of the Golgi might occur during cell division.
Warren’s experiments have shown that the vesicles are tethered during their journey within the Golgi stack by long fibers that seek out the next bit of cisterna. Warren and his group identified a docking protein, GM130, that allows the vesicles to find, then fuse with the next cisterna along the chain. They theorized that if the docking did not occur, then the tiny vesicles would continue to form (and to accumulate within the cytoplasm), but would not be able to fuse. Using microscopy as well as biochemical assays, they showed the process by which this massive fragmentation occurs and by which the thousands of vesicles are distributed nearly equally throughout the dividing cell. [To view animations of this process on the Web, see http://www.lif.icnet.uk/axp/cb/.]
What makes the work especially exciting, says Warren, who was elected a fellow of the Royal Society in May, is the confluence of disciplines that are being brought to bear on these and similar scientific puzzles. “It brings together traditional cell biology, biochemistry, molecular biology and, increasingly, structural biology,” he said. “We’re all now talking the same language.”
Although Warren deflects questions about the ultimate practicality of the knowledge he is uncovering (“it’s hard to say — these are just fundamentally interesting cell biologic processes,” he demurs when pressed), others are quick to point out the potential applications to medicine.
“The ability of cells to grow and divide, along with defects in those pathways, are the fundamental events that give rise to cancer,” said James E. Rothman, Ph.D., vice chairman of Sloan-Kettering Institute in New York City and chair of its cellular biochemistry and biophysics program. “What Warren has done is to help us understand the process by which a cell distributes its inner parts. The reason he is so well known and had such a broad impact is that his findings in this area have enlightened many other areas of cell biology.”
Warren and his group also have identified the mechanism by which the crucial docking protein is prevented from pulling in vesicles. Further, they’ve demonstrated that the regrowth of Golgi in the newly formed daughter cells requires the presence of several key molecules and that another protein called GRASP65 is involved in stacking the reformed cisternae to reconstitute the new Golgi complex.
Warren and his colleagues will look for additional proteins involved in the formation of the Golgi and attempt to answer another puzzling question: As the cell prepares to divide, how does the Golgi know to precisely double its mass and distribute that mass equally between the two daughter cells? “We can explore these questions in great detail biochemically — we grow Golgi beautifully in vitro,” Warren said. “By breaking it down and reconstituting it in the test tube, we can see what components we need to add to make the Golgi grow.”