Albert Lasker
Basic Medical Research Award

Today, we honor Jim Rothman and Randy Schekman for unmasking the traffic signals that direct the assembly line of the cell. Each cell is a factory that manufactures thousands of proteins every minute and exports them to the outside world. These proteins are assembled in steps that must be performed in the proper order. To organize this assembly, cells use workstations specialized for each chemical reaction. This solves one problem but it creates another: how to transport partially finished proteins from one workstation to another, and how to make certain that each workstation has the right amount of energy, supplies, and worker enzymes.

The work stations are called organelles. They are like balloons filled with enzymes enclosed by a membrane. If you were reduced to microscopic size and were standing inside a human cell, you would be surrounded by hundreds of these organelles floating in space. Every so often you would see a small bubble form on the surface of one organelle. The bubble is called a vesicle and its formation is called budding. The budding vesicle is filled with partially assembled proteins that must be carried to the next organelle. The vesicle detaches from its mother organelle and moves to a target organelle where it sticks like Velcro. Next the membranes between the vesicle and the organelle dissolve and the contents of the vesicle are injected into the organelle. This is called membrane fusion.

The great pioneers of cell biology—Palade, Claude, Porter, and DeDuve—discovered the organelles and mapped their order in the chain. Günter Blobel showed how nascent proteins are inserted into the first organelle. But a huge problem remained. How do the transport vesicles form? How do they select cargo so as to carry the products and exclude the worker enzymes? What Velcro attaches the transport vesicles only to the next organelle and not to one that is too early or late?

In the 1970s, when Rothman and Schekman began their work, most scientists despaired of solving these problems. The issues were too complex for classic biochemistry. The classic biochemist would crack open the cells, isolate vesicles, incubate them in test tubes, and measure the budding and fusion. The responsible proteins would be fractionated and the components would be isolated by biochemical complementation. This reductionist approach was used by Arthur Kornberg, when he made genes in a test tube. But even Kornberg would not dare attack a problem as complex as vesicle budding and fusion. This process must depend on the geometry within the cell. Vesicles bud and fuse at defined positions. Crack open the cell and you lose the geometry.

Up stepped Jim Rothman, a brand new biochemist at Stanford. Jim had the three ingredients essential for scientific success: training, environment, and youth. First: training. Rothman was suckled on membranes—as an undergraduate at Yale, as a graduate student at Harvard, and as a postdoctoral fellow at MIT. Second: environment. Rothman joined Kornberg's department which was built on the premise that no problem in biology is too complex for biochemistry. Third: youth. Rothman was young and fearless—what's more he had an idea.

A biochemist lives and dies by his assay. Rothman thought of a clever assay to measure vesicle budding and fusion in a test tube. His assay overcame the biggest problem—specificity—which arises because membranes are fragile. If you treat them too roughly, the organelles will burst and fuse at random, creating chaos that is impossible to sort out from the true cellular event.

Rothman's assay used mutant animal cells that were generated by Stuart Kornfeld at Washington University. These cells are missing an enzyme that is necessary to complete the assembly of secreted proteins. The proteins accumulate within organelles in a partially finished form. From these cells Rothman obtained organelles that contained partially finished proteins. He mixed these organelles with organelles from normal cells in a test tube. If vesicle budding and fusion occurred, then the unfinished proteins would move to the normal organelles and their assembly would be completed. And that is what Rothman observed. Directional movement in a test tube. The assay wasn't robust, but it allowed the purification of the first enzyme that was required for the fusion process. Like many revolutionary advances, Rothman's work was met with skepticism. Most cell biologists clung to the notion of vitalism—true vesicle movement could occur only in living cells. Rothman dealt a death blow to vitalism.

Before I go to the next chapter, I must introduce Jim's alter ego, Randy Schekman, who also had the three attributes of success. First: training. Randy earned his Ph.D. with Arthur Kornberg and we've already heard what he stands for. He then went to San Diego where he studied with Jonathan Singer, a master of membranes. Second: environment. Randy began his career at Berkeley, where biochemistry was dominated by Daniel Koshland, Jr., himself a Lasker awardee, and a firm believer in the power of biochemistry to unlock all mysteries. Third: youth. Schekman was young, and like Rothman he had an idea and the courage to try it out.

Schekman, like Rothman, was captivated by vesicle budding, targeting, and fusion. So far these two sound like twins. Both were trained in membrane chemistry, both were exposed to Kornberg and both began their careers as Bay Area biochemists in the late 1970s. Both also collaborated with Lelio Orci, the great microscopist of Geneva. But there was an important difference. Instead of tackling this problem through biochemistry, Schekman decided to use genetics. He chose a simple organism, a yeast. The biochemist works with broken cells in test tubes. The yeast geneticist works with living cells in Petri dishes. The geneticist's approach is more conservative because it does not deny vitalism. But Schekman's approach was bold in another way. It assumed that vesicle budding and fusion in a lowly yeast would resemble the process in human cells. After all, we don't study yeast because we want to make better beer or bread. We study yeast to learn something about ourselves. When Schekman started, there was little basis to assume that the process in yeast would be anything like the process in animal cells. The organelles hadn't been described microscopically in yeast. Just as Rothman proceeded on the world-view of Kornberg and Koshland, Schekman followed the precedent of Lee Hartwell, another Lasker awardee, who used yeast genetics to dissect the cell cycle and found principles that apply to all cells, including humans.

Like the biochemist, the geneticist needs an assay. Schekman conceived a brilliant and simple one. The challenge was to isolate mutant cells with defects in vesicle budding and fusion. This would unmask the crucial genes. Schekman realized that defects in budding or fusion would jam the cell's assembly line. The exported proteins would accumulate as unfinished goods. This buildup would make the cell heavier. Mutant cells could be separated from wild-type cells by centrifugation. Using this simple trick, Schekman and his students identified more than 70 genes required for vesicular traffic in yeast. These genes encoded proteins. Using classic genetic complementation, they determined the order of each protein in the yeast assembly line.

By 1980, Rothman and Schekman had both established their systems, and now the game began. For the next 20 years, the two played leapfrog. First one would discover a protein and then the other would use that information to discover the next protein. Each would vault to the next breakthrough on the haunches of the other. In a moment you will meet these men and you will note that one has the tougher task in leapfrog.

I don't have time to outline the individual accomplishments of Jim and Randy. They are monumental. Let me summarize by saying that we now have a clear understanding of the mechanisms by which protein cargo is sorted into vesicles, by which vesicles bud from organelles, by which they choose their proper target organelles and fuse with target membranes. These mechanisms are so fundamental that they are conserved from yeast to humans. This conservation allowed yeast genetics to play leapfrog with mammalian biochemistry.

The implications of this work are broad and deep. It gives us the instruction book for the assembly of a cell. It tells us how organelles are organized. The work teaches us how cells manufacture and secrete proteins like insulin and growth hormone that govern our metabolism. How brain cells discharge the chemical transmitters that mediate thought, feeling, and movement. How genetic defects in protein assembly cause human diseases like cystic fibrosis and heart attacks due to hypercholesterolemia. Indeed, we can no longer conceive of any cell, normal or diseased, without thinking of the fundamental processes that were discovered by Jim Rothman and Randy Schekman.

Let me close with an anecdote that shows how genes travel. First, I must tell you that Jim is a fiery and flamboyant orator who loves to challenge his audiences with audacious ideas. Randy is exactly the opposite. He is reserved and conservative, even fastidious. Both Jim and Randy had grandfathers who lived in neighboring villages or shtetls in Bessarabia in Eastern Europe. Rothman's grandfather was the village rabbi, a flamboyant orator, and Schekman's grandfather was the tailor. Genes cross oceans.