When Susan Taylor chose chemistry as her major during her freshman year at the University of Wisconsin, it never occurred to her that, in 1960, it was a bold move for a woman. She had always been an eager student and, from an early age, planned to go to medical school.
So, while she wasn't surprised to find herself in Madison, her nose buried in a chemistry textbook, most of her fellow, primarily male, students may have been a bit taken aback. But Taylor said she was inspired by her freshman-year professor, Charles Sorum, who made it seem natural for her to enter the honors program.
Forty years later, Taylor is still a standout in the field of biochemistry. She and her colleagues at the University of California, San Diego, have made great strides in unlocking the structure, dynamics, and localization of cyclic adenosine monophosphate–dependent protein kinase, or protein kinase A (PKA).
Cyclic AMP (cAMP) is an ancient molecule that first appeared in bacteria and is conserved in humans. cAMP is a universal signaling molecule; it translates signals from the cell surface and converts them into a biological response inside the cell. cAMP stimulates PKA, which controls the activity of other proteins through phosphorylation, a process that adds phosphate groups to those proteins. Phosphorylation may activate a protein, inhibit its activity, or cause it to bind to other proteins.
The mysteries Taylor and her colleagues have unraveled by probing the structure and function of PKA have broad implications. Ubiquitous throughout the body, PKA is one of the cell's most important signaling components. It helps regulate memory, development, cell growth, and cell death. When protein kinases go awry, they cause many diseases, especially cancers. The same is true for PKA. Defects in PKA are associated with immune disorders and also with many diseases such as cardiac disorders and cancers.
The kinase superfamily, for which PKA is a prototype, has become fertile territory for drug companies seeking oncology therapies. The anticancer drugs imatinib (Gleevec) for chronic myeloid leukemia and gastrointestinal stromal tumors and geftinib (Iressa) for non–small cell lung cancer are kinase inhibitors that have been both therapeutic and financial successes.
Taylor notes that PKA was the first kinase to be sequenced and cloned (she was not involved in either), and it is easily expressed, purified, and manipulated. It was also the first in the kinase family to be crystallized. In that process, x-ray beams are shot through the enzyme crystal, and the refractions are used to calculate a three-dimensional structure, making it possible to study how each part of the molecule works, and how all the parts work together.
In 1991, Taylor and her lab solved the crystal structure of PKA's catalytic subunit. The general structure of the catalytic subunit (the active kinase) is conserved throughout the kinase family, so it was the "Rosetta stone" for all the kinases, says Taylor. With a map of the catalytic subunit's structure, Taylor and her colleagues were able to determine how PKA works to phosphorylate proteins in the cell.
But they did not understand how PKA's regulatory subunit inhibits the catalytic subunit in the absence of cAMP. It was not until 2005 that Taylor and her colleagues could explain that phenomenon by solving the structure of the inactive complex. That allowed them to understand how the catalytic unit is inhibited in cAMP's absence and how it is activated by cAMP. With this, they finally understood just how PKA works.
Taylor has also come to understand how PKA transfers that phosphate from ATP to other proteins.
Protein kinases are also "a highly evolved example of allostery," said Taylor, noting that kinases bind to many other proteins and use their entire surface to communicate back to the area on the molecule where catalysis will take place. Her lab is using this knowledge to extrapolate general rules of allosteric regulation for protein kinases—how these enzymes are regulated by molecules that bind to sites that are outside of the active site.
When she started out in chemistry, kinases were far from her mind. She was determined to go to medical school, but during her junior year, she met her future husband, Palmer Taylor. He was finishing his Ph.D. at Madison and was already slated to go to the National Institutes of Health for post-grad work. So, Taylor applied for graduate school at nearby Johns Hopkins University.
They were married after her first year at Hopkins. "It was a very rash thing to do by today's standards," said Taylor, noting that they had been dating less than a year when they got engaged, and that it steered her onto a different professional course. But, she added, "In retrospect, it was the best decision I ever made, and it only changed my career path in positive ways."
When Palmer, a pharmacologist, was invited to join a new molecular pharmacology program in Cambridge, England, Taylor landed a fellowship at the prestigious Medical Research Council Laboratory of Molecular Biology. The institute's researchers—including Fred Sanger, Max Perutz, and Francis Crick—were among the best in their respective fields at the time.
Here she began working with Brian Hartley on proteins. "Once I started working on proteins, I didn't want to work on anything else," said Taylor.
The journey since has been intriguing, she says. After completing her MRC fellowship in 1970, she again followed her husband, this time to UC San Diego, where she accepted a fellowship with Nathan Kaplan, who was sequencing lactate dehydrogenase (LDH), and began a collaboration with Michael Rossmann at Purdue.
Rossmann had solved the crystal structure of LDH, but did not have the sequence. This was Taylor's job. She would visit his lab at Purdue, stand inside his giant, 5- by 6-foot model and try to match the sequence to various parts of the molecule. The aim was to eventually define the chemical features of the structure and thereby understand the function.
She soon secured a faculty position in UCSD's chemistry department, and not long after, began working with PKA. Kaplan "put a paper on my desk one day and said 'this is an interesting protein. You should think about working with it,'" Taylor said. And she did.
Even though she fell into working with PKA and proteins, Taylor has never regretted deviating from her original chosen career path. "Sometimes the most important defining things in your life you don't necessarily plan," she said. As a result, she said, "I'm having more fun."
And along the way, she had three children, which she considers her best experiments. They now have given her two grandchildren, setting her off in yet another new direction.