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FUNDAMENTAL RESPONSE If you want to answer the big questions in life, you’ve got to think and start small. That’s one reason why Gerald Hazelbauer, professor of biochemistry/ biophysics, works with the bacterium E. coli, as do many other scientists. by Mary Aegerter Hazelbauer’s work with E. coli began in the 1960s, when he was a graduate student in the lab of Julius Adler at the University of Wisconsin. Adler had been fascinated since childhood with the question of how organisms sense and respond to their environment. As a scientist, he looked for and found a system that would lend itself to the study of this question. He found it in chemotaxis by E. coli, a behavior in which these bacteria sense small molecules in their external environment and move in response to them. Scientists had already shown that bacteria can be attracted to a favorable chemical environment such as one containing the sugar galactose. Conventional wisdom held that bacteria were attracted as a result of using the galactose, breaking it down for food or energy. Adler, however, suspected that bacteria sensed the galactose by recognizing it, rather than by using it. He also realized that he could test his hypothesis with the newly developing tools of molecular biology. Adler’s lab was the first to study bacterial chemotaxis with these modern approaches. When Hazelbauer arrived, some of Adler’s researchers were trying to determine how the bacteria move. Others worked on how information about the presence of galactose was sent from the bacterium’s surface to the part of the bacterium that governed movement. Still others studied how the bacteria adapted to a new situation such as an increased concentration of galactose. Hazelbauer’s job was to find the means by which the bacteria recognized galactose. Adler reasoned that if the bacteria recognize galactose, then there must be something on the bacterium’s surface that does that recognizing, something that today we’d call a receptor. He further reasoned that it was most probably a protein, and he told Hazelbauer, “If you can find that protein, you can get a Ph.D.” In the 1960s this was a considerably more formidable task than it would be today. “The idea that there were receptors was around then,” says Hazelbauer, “but no one had identified one yet.” Hazelbauer found his protein receptor, not without some frustration and the proverbial stroke of luck. He also got his Ph.D. Hazelbauer spent additional time in Adler’s lab determining that there were similar receptors on E. coli for small molecules other than galactose. Then he went to France and worked on a different receptor for two years, before he returned to the galactose chemotaxis receptor and the question that had perplexed his thesis committee: how can a water-soluble protein receptor, which is what he found, cause information to move across a membrane that is not water-soluble? The answer would be that it does so by interacting with another protein, one that is not water-soluble, says Hazelbauer. He identified that protein. Since that time, Hazelbauer has concentrated on learning how the receptor works. “And I’m still trying to figure that out,” he says. The E. coli chemotaxis receptor for galactose is a complex of three parts. On the outside of the bacterial membrane is the section that recognizes galactose, the section Hazelbauer identified as his thesis project. On the inside of the bacterial membrane is a section that interacts with another protein inside the bacterium. In-between is the second of Hazelbauer’s discoveries, the section that spans the bacterial membrane and is not water-soluble. When the external section of the receptor recognizes and binds galactose, a signal passes through the rest of the receptor and causes sequential changes in two proteins inside the bacterium. The first protein is called a kinase and sits next to the receptor. In the absence of a signal, the kinase activates a second protein, the regulator, which then leaves the receptor complex and interacts with the flagellar rotary motor’s gear shift. (See sidebar.) This causes the flagella to turn clockwise and the bacterium to tumble. In the presence of a signal from the receptor, the kinase cannot activate the regulator protein. Thus, the flagella continue to turn counterclockwise, and the bacterium does not tumble. Instead it swims smoothly in the direction of the galactose. The two-protein kinase-regulator system is ubiquitous. “All bacteria sense their environment with this type of sensing system,” says Hazelbauer. Salmonella species know they’ve entered one of your cells using this system. The soil bacterium Rhizobia is able to find a root hair and begin to make a nitrogen-fixing root nodule using this system. And species other than bacteria use this system, says Hazelbauer. “Two- thirds of all life on earth use it.” One place the system has not been found to date is animals. This makes it of interest to those developing antibiotic drugs that kill bacteria without harming patients. Hazelbauer’s attempt to understand how the chemotaxis receptor works has meant learning as much as possible about the receptor’s structure and organization. He discovered that the part of the receptor inside the bacterium contains a number of separate sites that are modified as the cell adapts to persistent stimuli. He also deduced the three-dimensional structure of the receptor’s transmembrane domain. His work is now focused on determining how the individual helices that make up the transmembrane section move with respect to each other. This movement conveys a signal to the inside of the cell that galactose is present on the outside. E. coli use their galactose chemo-taxis receptors to determine whether the surrounding concentration of galactose is changing. The signal that passes through the receptor when galactose binds does more than activate the two-protein sensing system inside the E. coli and change flagellar rotation. It also causes a modification at specific sites in the receptor’s interior section. The signal’s passage is rapid, for changes in flagellar rotation can be seen within about 0.2 seconds. The modifications take longer to occur. It appears that the bacteria periodically monitor both the modification sites and galactose binding. The modification sites provide information about the concentration of galactose some few seconds in the past. The galactose binding provides information about the current concentration. If the two differ, then the concentration is changing. If the bacterium were bigger, it might instead compare the concentrations of galactose at its front and back ends. But again, its small size is a handicap. Any difference in the galactose concentration measured at the bacterium’s two ends would be no larger than the statistical variation found in a uniform concentration. Thus the bacterium substitutes comparisons over time for comparisons over distance. When the concentrations are changing, the bacterium will send a signal to the flagella’s gear shift, and the bacterium’s movement will be adjusted to reflect that difference. If the concentrations are the same, however, no adjustments are necessary, and no signals are sent to the flagella. The bacterium is adapted to its surroundings. And just as E. coli adapts to its surroundings, we adapt, for like E. coli's sensory system, ours are designed to detect change. If we walk into a room with a bad smell, for example, we notice it. If we remain in the room for several minutes, we notice it much less or not at all. Current evidence suggests that our adaptation may not be all that different, at the molecular level, than that of E. coli. Membrane-spanning proteins in nerves are modified in the course of adaptation in a way that is analogous to the galactose receptor. “These changes in membrane proteins appear to be a first step or at least a part of the first step of our storing information,” says Hazelbauer. We, again like E. coli, have a memory. Much progress has been made since the 1960s in learning about how organisms sense their environments. “We set out then to study a fundamental issue in biology with the newest and most powerful of molecular tools,” says Hazelbauer. “These tools have made molecular biology one of the great intellectual success stories of the second half of the 20th century. We’ve been able to learn a great deal about how life works at the level of the molecules.” Mary Aegerter writes about science and hiking from Uniontown, Washington. |
