Calm in the STORm

A highly charged atmosphere seems to suit Hall, who thrives being at the center of the action, if not the storm. Throughout his career, Hall has taken on risky projects that have paid off big—the biggest of which was the identification and characterization of TOR, the protein target of the immunosuppressive drug rapamycin. In addition to cloning TOR from yeast, Hall has helped to sort out the pathways through which TOR regulates cell growth—pathways that are conserved in all eukaryotes, including humans. “What started out as a search for the target of rapamycin has developed into a spectacular collection of discoveries that a lot of labs have since climbed onto,” says Scott Emr of Cornell University in Ithaca.

Princeton’s Tom Silhavy agrees. “TOR is now a growth industry,” he says. “And a highly competitive one at that.” Of course, it wasn’t always smooth sailing. “In the beginning, TOR wasn’t as popular as it is today,” says former student Tobias Schmelzle of Novartis Institutes for Biomedical Research in Basel. “There were low phases in the lab when it was hard to publish. But Mike would not walk away. Now there are few people who don’t know him as the founding father of the TOR field.”

Hall was born in San Juan, Puerto Rico and grew up in South America, where he had his own lab building in the back yard. “There was this old, run-down shack that I painted white and painted ‘lab’ on the door,” he says. “But the only ‘experiment’ I really did there was to make gunpowder.” That gunpowder didn’t have nearly as much bang as the recombinant DNA technology he started tinkering with as an undergraduate at the University of North Carolina. His mentor Marshall Edgell even brought Hall along to a meeting at NIH, chaired by then-chairman Donald Fredrickson, in which scientists attempted to develop guidelines for safely working with recombinant DNA. “We all sat around this big oval table and we knew it was history in the making,” says Hall. “That really got me hooked.”

At Harvard, Hall took up residence in Silhavy’s lab, which at the time was studying protein secretion in E. coli. But Hall had other plans. “I think I’ve always had a kind of contrarian spirit,” he says. “Protein secretion was interesting, but I didn’t want to work on it because that’s what everyone else was doing.” Instead Hall focused his attention on a set of outer membrane proteins called OmpF and OmpC. And he discovered another pair of proteins, called OmpR and EnvZ, which together form a signaling system that regulates the production of OmpF and OmpC. “So he wound up characterizing the first two-component regulatory system in E. coli,” says Silhavy—the first such system in any organism, notes Hall. The resulting papers, says Silhavy, “are among my most highly cited.” Altogether, Hall published 13 articles with Silhavy, including six first-author papers, a Nature letter, and several large reviews. “That’s exceptional for a graduate student,” notes Emr, who was a student at the next bench.

Hall kept the ball rolling as a postdoc at the Pasteur Institute—and again during a second postdoctoral fellowship at the University of California, San Francisco. At UCSF he worked with Ira Herskowitz, who was also famed for “using elegant genetics to get at complicated regulatory pathways,” says Hall. In keeping with his contrarian tendencies, Hall declined to work on the topic that concerned the rest of the lab: how yeast responds to pheromone signals. Instead, he opted to examine how nuclear proteins are targeted to the nucleus after being synthesized in the cytoplasm. At the time, people figured that they got there by diffusion. “Then, if they bound something they stayed there,” says Hall. “If not, they just diffused back out. This was the accepted model.”

But Hall thought there must be something less passive governing something so important. He was betting that these proteins contain some sort of signal that directs them to the nucleus. Not everyone agreed. “George Palade—a Nobel Laureate with a reputation of being the kindest gentleman in science—came to San Francisco while I was working on the project,” says Hall. “And when I told him what I was doing, he looked at me and said, ‘That’s the stupidest thing I’ve ever heard. You should get out of science.’ But I was so heavily invested, I couldn’t back out.” And through a series of nerve-wracking experiments, which included producing a set of precisely engineered fusion proteins and developing immunofluorescence techniques for tracking these hybrids in yeast, Hall discovered his nuclear localization signal, work published in Cell in 1984.

“That’s just his style,” says Emr. “To counter the prevailing dogma. I think he almost takes it on as a challenge.”

Hall took the problem of nuclear localization with him when he started his own lab at the University of Basel’s Biozentrum in 1987. He was hoping to find the receptor protein that recognizes the localization signal he’d discovered. “The work was failing miserably and we were getting rather desperate,” he says. So desperate they turned to drugs: the immunosuppressive drugs cyclosporin A and FK506.

When T cells recognize a foreign antigen, they proliferate. And these drugs were known to block the signal that spurred activated T cells to divide. What that signal was, no one knew. But Hall thought perhaps the drugs kept a key transcription factor from entering the T-cell nucleus following antigen stimulation. “If that were the case,” says Hall, “we thought we could use these drugs to identify the nuclear import machinery.”

Exposing yeast to cyclosporin and FK506 did inhibit their ability to divide—but the effect was fairly mild. Then along came rapamycin. Hall knew a scientist at Sandoz, Rao Movva, who was interested in identifying the target of this new immunosuppressive drug—which Hall hoped to try on his yeast. “That connection was absolutely critical because you couldn’t buy these compounds back then,” says former postdoc Joseph Heitman, now at Duke University. “Rao came by with an Eppendorf tube and said, ‘This is the world’s supply of rapamycin.’ It was about a milligram. Enough for one Petri-dish worth of experiments.”

With that first dish, Heitman and Hall confirmed that rapamycin could keep yeast cells from dividing—and that it did so by binding to FKBP, a protein they had previously identified. Get rid of FKBP and rapamycin has no effect. They then screened yeast to find out what proteins interact with the rapamycin-FKBP complex—and discovered the genes encoding the Targets of Rapamycin, TOR1 and TOR2. In addition to being a handy acronym, Heitman says, “the name TOR has a more romantic origin. Basel is an ancient city that was ringed by a wall whose gates would be closed at night. And the German word for ‘gate’ is Tor. We envisioned these proteins as gates to the cell cycle.”

In fact, they are gates to the broader process of cell growth, as Hall went on to prove. First, he found that TOR controls general protein synthesis. Next, that it regulates ribosome biogenesis. “Then we realized it controls many processes that ultimately impinge on the accumulation of mass,” says Hall. The trouble was convincing everyone else. “It was a very dark period in the lab, because nobody believed us,” he says. And getting the key paper published was a challenge. “It just kept getting bounced.”

The problem, says former student Stephen Helliwell of Novartis, was that the manuscript crossed too many fields. It included figures on cell cycle, protein translation, and cell signaling, among other things. “So no single reviewer could handle it,” he says. “It was a total nightmare.” After nearly 2 years of floating around, the paper landed on Lee Hartwell’s desk and was rapidly accepted for publication in the fledgling journal Molecular Biology of the Cell in 1996. “Mike never gave up,” says Helliwell. “And he gave off this aura that it would all be okay. He had this quiet confidence and tenacity that gave you the feeling that he would succeed. That paper was a real breakthrough and has stood the test of time. So there’s no question we were on the right track.”

Hall and his team continued to pave the way, identifying the proteins with which TOR interacts and untangling the pathways through which TOR controls cell growth. As other labs have identified TOR and its signaling partners in mammals, Hall says, “it’s become clear that TOR is at the center of a highly conserved pathway that controls cell growth in eukaryotes, from unicellular organisms all the way to humans. It just doesn’t get any more fundamental than that.”

Now the Hall lab has taken to tackling TOR in mammalian systems in the hopes of understanding how TOR controls growth in a whole animal—and how TOR signaling pathways play into health and disease. In a study published last year in Cell Metabolism, Hall and his colleagues show that knocking out one of the two functionally distinct TOR complexes in adipose tissue makes mice fit and trim. “These mice can eat whatever they want and they don’t get fat,” he says. “And they’re metabolically healthier than wild-type mice.”

Given that others have recently linked TOR with the longevity-boosting effects of calorie restriction, Hall is waiting to see whether these skinny mice will also live longer—work he hopes to publish before someone else does. “For 10 years we dominated the field. But it’s become really huge,” he says. “Now you can’t pick up a journal without seeing a TOR paper. So we’re getting scooped a lot more these days. But it’s been a really nice ride.”