Divide, Conquer

Glotzer had been working in Kirschner’s lab on the problem of cell-cycle control. At the time, researchers knew that cyclin proteins had to be degraded to allow cells to progress through mitosis. But no one knew how. So the Saturday before the lab-selection deadline, Kirschner called Glotzer into his office. “He said, ‘Michael, it would be really great if you could come to the lab and work on cyclin degradation.’ And then it hit me,” says Glotzer. “That’s exactly the type of problem that I really like: We knew that cyclin degradation happened. We knew it was important. But mechanistically, we didn’t have a clue. We didn’t even know how to approach it. So at the last minute I totally changed my mind and decided to go to Marc’s lab. The rest, I guess, is history.”

Indeed, Glotzer’s finding that cyclins are marked for destruction by a protein tag called ubiquitin was a landmark discovery. The resulting Nature paper, which came out in 1991, “was one of the half-dozen most important papers ever published in the cell-cycle field,” says Kirschner, now at Harvard Medical School. “Up until that time, most people thought that ubiquitin was only involved in waste disposal: Proteins get old, they get tagged and removed. But here was one of the most exquisitely timed cellular transitions making use of the ubiquitin system. That discovery equaled any other experiment in the field in terms of significance and importance.”

Although Kirschner describes those accomplishments as “hard to beat,” Glotzer has continued to take on big questions that are tough to tackle—including how cells split in two. “It was the same kind of problem,” says Glotzer. “Here again, you could watch movies of cells dividing, but nobody knew how the spindle tells the cell where to divide.” To that end, Glotzer has dissected cytokinesis, the last stage of eukaryotic cell division. And he’s discovered, among other things, that two redundant mechanisms govern where a dividing cell places its cleavage furrow, the point where the daughter cells physically part ways.

“It’s a fundamental step in cell division, and one of the holy grails of cell biology to understand the cues that position the cleavage furrow,” says Bruce Bowerman of the University of Oregon. “Other people have made contributions, but Michael has really led that field.”

Glotzer’s path to the lab was almost as complex as the pathways he would go on to study. After losing interest in academics in high school, Glotzer wound up attending a small liberal arts college in New London, Conn. “But I thought it was too much of a party school,” he says.

The search for something different landed him at the little-known College of the Atlantic in Bar Harbor, Maine, where, as a visiting student, Glotzer decided to work as an intern with his friend’s father at the nearby Jackson Laboratory. Over the following year—eventually working full-time in the lab—Glotzer helped Les Kozak clone a gene that encodes a mitochondrial uncoupling protein in mouse brown fat. In the fall of 1984, he transferred to the University of California, Berkeley as a junior.

At first, Glotzer flirted with the idea of focusing on math. “I thought it was fun,” he says. “I liked that you could do a problem and get an answer and know whether you were right.” But being at Berkeley cured him of that. “I realized there were some amazingly good mathematicians there. And I wasn’t one of them,” laughs Glotzer. “I also realized that I liked the team effort of working in a lab, compared to the more solitary task of a mathematician.” By the time he hit UCSF as a graduate student, he was ready for a real biological challenge.

Enter the problem of cyclin degradation. Progression through the cell cycle involves the cyclical accumulation and elimination of the regulatory cyclin proteins. To address the question of why and how they disappear, Glotzer set up an in vitro assay to see what happens to radiolabeled cyclins when they’re exposed to extracts from Xenopus eggs made during different stages of the cell cycle. Running these mixtures out on a gel, he could watch the labeled cyclin disappear with time—but he couldn’t see any obvious breakdown products. “I remember even having a figure on a poster titled: ‘Cyclin is degraded with no detectable intermediates,’” says Glotzer.

Then came summer vacation. Before leaving, Glotzer set up an experiment in which he was comparing wild-type cyclin with a nondegradable cyclin that had a mutation in a region known as the “destruction box.” When he ran the samples out on a gel, and developed the film, he saw the expected results: the wild-type cyclin got degraded and the mutant protein was stable. He could have set it aside, but instead he slapped another film onto the gel, stuck it in the freezer, and headed off on a weeklong hike.

“When he came back, he had the world’s ugliest exposure of cyclins,” says Kirschner. “The bands were an inch thick, smeared across the film. But instead of throwing it away, he noticed that above the cyclin band was an equally spaced ladder of proteins going up the gel. By the time he showed the results to me, he had puzzled it out and come to the right conclusion.” That ladder of proteins represented cyclins with progressively more and more ubiquitins attached to them—hence their running more slowly on the gel. And it was the extra-long exposure that allowed him to see this ladder of ubiquitinated products, which under normal conditions was too faint to detect.

“The demonstration that cyclins are regulated by ubiquitination solved the mystery of what makes cyclin levels go up and down during the cell cycle,” says Bowerman. With more than 1500 citations, “that Nature paper is one of the most cited papers I know,” he says.

During his postdoc at the European Molecular Biology Laboratory in Heidelberg, Glotzer started off with a similar, build-from-the-ground-up biochemical approach to studying cytokinesis: He mixed together purified components—some actin and myosin along with bits of membrane—in an attempt to reconstitute the larger process. But the system was tough to pin down. “The structure that powers cytokinesis is transient,” notes Bill Bement of the University of Wisconsin. “It’s not there, then it magically appears, and then disappears”—which makes it almost impossible to purify.

Assembling cytokinesis from scratch “was like trying to hit a grand slam when at that point what we really needed was to get on first base,” says Glotzer.

That first hit came from a collection of mutant C. elegans that Glotzer and fellow postdoc Pierre Gönczy screened for embryos that were unable to complete their first cell division. “We would spend 8 or 10 hours a day in the microscope room, imaging these embryos every 5 seconds and seeing where development would go wrong,” says Gönczy, now at the Swiss Federal Institute of Technology (ETH) in Lausanne. In one mutant, a cleavage furrow would form, but then regress. Glotzer cloned the gene, which he called cyk-4, and found that it encodes a protein that stabilizes the central spindle—a structure that helps determine where the cleavage furrow will form.

The cyk-4 mutants, he then realized, have the same phenotype as worms that lack a gene called ZEN-4, which encodes a kinesin motor protein. Working with postdoc Masanori Mishima at his newly opened lab at the Research Institute of Molecular Pathology in Vienna, Glotzer discovered that CYK-4 and ZEN-4 come together to form centralspindlin, a complex that regulates cytokinesis.

“I was a bit skeptical at first,” says Mishima, now at the University of Cambridge. The early experiments—which showed that the two proteins could be coprecipitated from a crude cell preparation—“were not super beautiful,” he says. But when Mishima produced the two proteins by translating them in a test tube, he found that they did interact. “Michael suggested that technique and it seemed to work magically,” he says.

Perhaps what’s truly magical is Glotzer’s ability to use genetics, microscopy, and biochemistry to probe this fundamental cell biological process. “Michael doesn’t take a linear approach to his science,” says former postdoc Alisa Piekny of Concordia University. “He uses multiple organisms and multiple techniques, which is why he’s able to make such strong headway in these different fields. He also enjoys what he’s doing.”

Bowerman agrees. “He’s ambitious, and I’m sure he has as much of an ego as any successful scientist,” he says. “But he also stands out as being very playful, fun-loving, and accessible.” That attitude benefits the science—and even the community. “Michael invited me to conference he hosted back in the mid-1990s, and it was the best meeting I’d ever been to,” says Bowerman. “He got up in his cut-off jeans and T-shirt and made some jokes. It got the meeting off to this very friendly, interactive start and made people feel at ease. Those were some of the best Q&A sessions I’ve ever seen at a conference.”

Glotzer has also penned a plethora of review articles. These thoughtful pieces not only keep biologists abreast of the latest work in cytokinesis, they put ideas out there for everyone to “shoot at,” says Bement.

And when it comes to his science, these days Glotzer uses his head more than his hands—except when it comes to calibrating the lab’s pipettors or fixing errant equipment. “Michael is definitely hands-on when something breaks,” says Piekny. “If there’s something wrong with the microscope, he’ll be in there pulling on wires and removing things. He’s technically very savvy.”

“My first instinct is to get in there and take things apart,” says Glotzer. So far that approach has served him well.