Growing up on a Wisconsin farm, Wayne Hendrickson developed a penchant for machines and a need to know about nature. These two themes have persisted as this biophysicist has invented new techniques for uncovering the structures of biomolecules and applied them to problems ranging from worm respiration to AIDS. "The possibility of understanding living systems in fundamental physical terms has always seemed powerful and also very gratifying to me," he says.
Hendrickson followed his studies of physics and biology at a small Wisconsin college with a doctorate in biophysics at Johns Hopkins University. In 1969, he accepted a position at the U.S. Naval Research Laboratory (NRL) in Washington, D.C., to work with Jerome Karle, who would win the Nobel Prize in Chemistry 16 years later. In 1949, Karle had mathematically solved the long-standing problem of how to determine the phase of an x-ray that is diffracted by a crystal. This information is needed to translate the pattern of spots that diffracted rays make on film into the locations of atoms in a crystal. Hendrickson was recruited to extend this phase work to the x-ray crystallography of large molecules, such as proteins. "The mathematical solution is very powerful and important, but it is inadequate to address large biological molecules," Hendrickson explains. "We had to come up with more experimental ways to do that."
About that time, synchrotrons became available as tunable x-ray sources. Hendrickson therefore decided to use x-rays of different wavelengths to obtain diffraction patterns from proteins. This innovation, called multiwavelength anomalous diffraction, or MAD, solved the phase problem for crystallized proteins. "The MAD method relieved us of the need to prepare variously derivatized crystals. And, especially with the use of selenomethionine substitution, in most cases it all but assures success from one single crystal," Hendrickson says.
Hendrickson worked mostly by himself at the NRL. "Complete immersion in research, free of distractions that prevail in academic institutions, helped me to form my own view of how to advance a research problem," he says. Among the other highlights of his 15 years at that institution was a pioneering computer program called PROLSQ, which aligns atomic models with the data collected by x-ray crystallography. Elements of this program are now incorporated into other computer programs still in use.
Wanting to be around more biologists, Hendrickson joined the faculty of Columbia University in 1984. "Moving to Columbia brought me many wonderful students and directed my attention to new biomedical problems, which shaped my future research," he says.
Signal transduction, which transmits information from a cell's environment to its interior, has been one of those problems. Hendrickson took the field in a new direction by looking at the structures of signaling proteins for clues as to how they might work. For example, his group obtained the first crystal structure of a signaling protein called tyrosine kinase, which is part of the cellular receptor for insulin. They also determined the structure of the tyrosine kinase associated with CD4, a cell-surface protein involved in cellular immunity, and of several other kinases. Many of these molecules are now important targets for the pharmaceutical industry because drugs that could block kinases might be useful in many diseases, particularly cancer.
Other projects have involved structural studies of respiratory pigments from worms, clams, and octopi; cell adhesion molecules; growth factors and their receptors; reproductive hormones and their receptors; and molecular chaperones that respond to cellular stress. "When we see things that no one has seen before," Hendrickson says, "it's like conquering mountains."
This intellectual mountaineering has produced additional advances in x-ray crystallography. Beginning in 1987, with HHMI support, Hendrickson developed a synchrotron beamline at Brookhaven National Laboratory that remains one of the most productive in the world. His earlier structure of crambin introduced a forerunner of MAD, which is now called SAD (single-wavelength anomalous diffraction). "SAD has the advantage of yielding structures from single data sets, thus reducing the required effort," Hendrickson says. MAD and SAD are now widely used in x-ray crystallography.
Hendrickson regards his studies of the human immunodeficiency virus (HIV) as his most important biological contribution to date. This work evolved from structural studies of the protein CD4, which participates in the immune response. CD4 sits on the surface of the T cell that is supposed to help destroy pathogenic microbes. This cell is the main target of HIV, and its numbers drop precipitously in AIDS patients.
Like the flu virus, HIV evades the immune system by constantly mutating its protein coat and hiding under layers of sugar. However, one of its sugar-proteins, glycoprotein 120 (gp120), must interact with CD4 before the virus can get into cells; therefore, the part that interacts with CD4 can't change. How, then, does the virus keep gp120 constant without revealing its identity to the immune system?
By studying complexes of gp120 and CD4 in solution, Hendrickson and colleagues uncovered HIV's cunning strategy. The surface that binds to CD4 does not exist in the absence of the cellular protein! But the exterior of gp120 is so flexible that when it meets CD4, disparate parts come together, like jigsaw-puzzle pieces, to make a surface that binds immediately to a second T cell receptor. This happens so fast that the immune system can't even glimpse the potentially antigenic surface.
Hendrickson showed that the energy needed to assemble the gp120 surface that interacts with CD4 is more than offset by the energy saved when that interaction occurs. Therefore when CD4 is present, the pieces are more likely to snap together than remain apart. "It is extremely thrilling when you have something that gives you a grasp on how something works that was just baffling beforehand," Hendrickson says. "In our business, answers come as a big surprise after quite a long struggle."
Hendrickson finds such struggles "absolutely engaging." At present, he's trying to learn more about the interactions between HIV and T cells. He also is studying heat-shock proteins, which help cells survive environmental stress. Moreover, his work on signaling molecules has led him to receptors for serotonin, one of the brain chemicals that are depleted in depression. "We think that understanding serotonin receptors in detail will give us additional insights into those kinds of disease situations," he explains. "We think we can best advance medicine by working at a very fundamental level to understand basic biophysical mechanisms."