Albert Lasker
Basic Medical Research Award

Award Presentation by Ira Herskowitz

One of the simplest but also one of the most profound questions to ask about biology is "How do you make a cell?" By that I mean, "How does one cell produce two cells?" A human skin cell divides to produce two cells like itself in about ten hours. A bacterial cell produces two bacterial cells in about 30 minutes, and a yeast cell produces two yeast cells in about ninety minutes.

In an intellectual feat as exciting as the athleticism displayed by the dynamic duo of Sosa and McGwire, Watson and Crick explained how the genetic instructions of the original cell are copied so that each of the daughter cells receives a precise replica of the original genetic information—a DNA double helix. That's why each of the daughter cells looks like the original cell: they both get the identical genetic information.

As breathtaking as Watson and Crick's explanation was, it didn't answer another fundamental question about cells: how do the daughter cells manage to receive the new set of instructions? Do the newly replicated DNA molecules, the chromosomes, just float into the daughter cells or get distributed randomly to these cells? No, of course not. Cells have precise machinery to distribute the genetic information so that each cell gets exactly one copy of each chromosome.

So, now we can see some of the richness of a deep problem in biology: How do cells choreograph not only copying their genetic information but distributing this information? The cells had better do the copying before the distribution, or there will be a mess. How do cells know in what order to carry out such processes? Studies by Lee Hartwell, Paul Nurse, and Yoshio Masui identified the molecular machinery and the logic of how this machinery works to program the sophisticated events involved in duplicating cells.

Here's another fundamental question: What tells a cell when to divide? Many cells in our body aren't dividing; they are just sitting around in a resting state, waiting for a signal to divide. What are these triggers? And what about cells that become deranged, so that they think they are getting a stimulatory signal when they are supposed to be resting? These are cancer cells. What is the molecular basis for their derangement? Once again, the studies by Masui, Nurse, and Hartwell identified the molecular machinery that lets us understand many of the derangements that we see in cancer.

Beginning in the 1800s, with observations of cells by microscopy, and continuing into the mid-1950s, it became clear that the cell cycle has observable phases: S—when the chromosomes are duplicated; M—when the chromosomes are distributed, and two gap phases between them. Some of the first clues about what governs the cell cycle came from experiments in the late 1960s when nuclei from one stage of the cell cycle were injected into the cytoplasm of cells in other stages or when cells in different stages of the cell cycle were fused with each other. These experiments revealed that there was something in the cytoplasm of the cell, some factor, controlling the behavior of the nucleus. Would it be possible to get one's hands on such molecules? That was the Holy Grail of the new cell biology. Our awardees worked on three different organisms—two different kinds of yeast and on frog oocytes, exploiting specific features of these experimental systems to find these cell cycle regulatory molecules, the Grail. It was a journey that took them and others in the field more than fifteen years, but what they found was enormously satisfying: All of their work converged to identify a particular piece of machinery, a protein kinase, that is central to regulating the cell cycle.

Lee Hartwell was exposed to microbiology and the power of genetic analysis at an early and impressionable age. As an undergraduate student at Caltech in Pasadena, he worked in Robert Edgar's laboratory, which pioneered the strategy of how to identify every gene in an organism. They used an organism that had only around 150 genes, a virus that grows in bacteria, and exploited a genuinely elegant strategy, the use of conditionally lethal mutants. These are mutants that can grow under some conditions, for example, at low temperature, but not under other conditions, for example, at high temperature. While a beginning assistant professor at the University of California, Irvine, Hartwell decided that he wanted to know how a eukaryotic cell works. So he chose the simplest possible eukaryotic organism, in this case a eukaryote with just a single cell, yeast, and proceeded to identify yeast genes essential for its growth, employing the same strategy that had been used in the Edgar laboratory—finding mutants that are temperature sensitive for growth. He found hundreds of mutants and dozens of interesting genes this way. A few years later, after Hartwell had moved to the University of Washington, he realized that some of these mutants had the striking property that they got stuck at a particular position in the cell cycle when they were incubated at high temperature. He could readily discern the step at which they got stuck because he was studying budding yeast, an organism whose daughter cells form by a process of budding: Cells with a small bud are in an early stage of the cell cycle, and cells with a large bud are in a later stage of the cell cycle. Studies of mutants like these led him to identify dozens of genes, including one, CDC28, that proved to be particularly interesting, and to define a small set of processes that were carried out in a particular order: Cells don't move on to step 2 until they complete step 1. Later work led to an understanding of how cells monitor the completion of these steps—they have molecular "checkpoints," where this monitoring takes place.

Paul Nurse had also become smitten by microbes during his studies as an undergraduate in Birmingham and began working on a different yeast, fission yeast, to learn about the cell cycle. Like Hartwell, Nurse isolated many temperature-sensitive mutants, which defined a variety of steps during the cell cycle of this organism. But his great insight was to appreciate the novelty and value of another type of mutant: not one that gets stuck in the cell cycle but rather one that speeds through the cell cycle inappropriately. These mutants had the interesting phenotype that the cells are small or, as he described them using local lingo, Wee. In the normal process of cell division, a cell gets longer and longer until it reaches a critical size, and then the cell pinches off to form two cells. Nurse reasoned that the Wee mutants are altered in this important decision-making step in the cell cycle: they pinch off to form two cells prematurely. He had six or so Wee mutants and sought to figure out which genes were altered in these mutants. The first five all affected a gene he called wee1. Considerable legend surrounds the sixth Wee mutant. This mutant was on a plate that got contaminated by some volunteer microorganism like bacteria or a mold, and rather than purify mutant #6 away from the contaminant and carry out the analysis to find that it, too, was a mutation in the wee1 gene, Nurse tossed it into the garbage. Something gnawed at Paul and he returned to the lab. Sifting through the garbage, he recovered the plate, and using standard microbiological procedures, he separated the mutant yeast from the contaminant. Much to his delight, this mutant did not have a mutation in the wee1 gene. Rather, it had a special alteration in the cdc2 gene, a gene that was already known because other types of mutations in it caused the cell cycle to become stuck. Discovering this special mutation in the cdc2 gene induced Nurse wisely to focus on this gene and how it is regulated. Focusing on the cdc2 gene proved to be just the right thing to do: In later studies, Nurse showed that it is so important that not only does budding yeast have a version of it (none other than Hartwell's CDC28 gene), but Paul fished out a human version as well.

Yoshio Masui's early work at Kyoto University in Japan was on development of amphibians—his Ph.D. thesis was on "Studies of the Effects of Lithium Upon Morphogenesis of Amphibian Embryos." Although frogs were excellent for classical studies of development, Masui sought to learn more about the molecules involved, and so he took a sabbatical from his position at Konan University. He came to Clement Markert's laboratory at Yale to learn about enzymes involved in development and spent six months studying particular enzymes from penguin embryos. He soon recognized that penguins were not an exceptionally propitious experimental system and, anyway, Markert told him he "should choose something less expensive to experiment with." And so he returned to amphibians. The key feature of frogs that Masui exploited was that their oocytes—progenitors of eggs—are naturally arrested at one stage of development. Further development of the oocyte, the process of "maturation," was triggered by addition of a hormone such as progesterone. In a series of remarkable experiments, Masui discovered that the cytoplasm of oocytes treated with progesterone contained an activity, "maturation promoting factor," that induces maturation in untreated oocytes. He had demonstrated an activity that could drive the cell cycle in these cells. To people who sought a molecular understanding of cell-cycle regulation, MPF was truly a Holy Grail. What was its molecular nature? What did it do? What controlled it? For a variety of reasons, it took more than 15 years before the molecular identity of MPF was identified, which occurred in a flurry of activity in the late 1980s. Astonishingly, a key ingredient of maturation promoting factor was, you guessed it—the frog version of cdc2 protein.

And so resulted one of the most remarkable convergences in modern science, a truly unifying discovery, in which it was seen that yeasts, frogs, and humans share universal machinery for regulating cell division.