Examining pond water through a microscope in junior high, Pamela Björkman was amazed to see creatures that were invisible to the naked eye. "Perhaps that's why I chose to work on discovering the structures of proteins, which are even smaller than the [organisms] one finds in pond water," says Björkman, who elucidates the three-dimensional structures of proteins that interact with other proteins and explores the functional implications of those structures.
Björkman studied chemistry as an undergraduate in Eugene, Oregon. But after she became a graduate student at Harvard University, she heard a lecture by the late Don Wiley (an HHMI Investigator at the time of his death), who was trying to solve the structure of an influenza virus protein. Realizing, for the first time, that protein crystallography can address major medical problems, she decided to enter that field.
During her postgraduate and postdoctoral studies with Wiley, Björkman made a major coup: she solved the structure of a protein that tells T lymphocytes whether a particular cell is healthy or infected with viral particles and therefore should be destroyed. This class of protein, called a human leukocyte antigen (HLA), sits on the cell surface, presenting an antigen—in this case, part of a virus—like a badge holder displaying a badge. But in the 1980s, scientists didn't understand how T lymphocytes could see antigen and HLA protein at the same time, though they had deduced that simultaneous recognition of both structures initiates killing.
Using x-ray crystallography, Björkman determined the 3-D structure of a particular type of HLA protein, HLA-A2, though eight trips to a synchrotron (a source of high-energy x-rays) in Germany were required. One of the protein's most striking features was an impressive groove on its upper surface. Surprisingly, the purified protein contained antigen in this groove, even though it had been purified from uninfected cells. Thus, the badge could be read by T cells as either "self" or "foreign," the researchers realized. Moreover, the antigen and top edges of the groove formed a contiguous surface. "So the T cell receptor just sits on that and sees chemical information from both [the antigen and HLA-A2]," Björkman says. The structure immediately revealed why both were recognized at the same time and suggested what goes wrong in autoimmune disease: T cells mistakenly recognize a self molecule for a foreign antigen. In a 1987 Nature paper, Björkman said the occupant of the groove was "probably a peptide," which later proved to be correct.
When Björkman established her own lab at Caltech in 1989, she began to look at relatives of class I major histocompatibility (MHC) proteins, the class to which HLA-A2 belongs. Her Web site now displays the structures of the ~25 proteins and complexes her lab has solved over the years. Surprisingly, not every protein uses the counterpart of its groove when it interacts with other proteins. Moreover, some of the proteins are not even remotely involved in immune functions.
One protein that has fascinated Björkman throughout her career is FcRn, the neonatal receptor for the Fc portion of an immunoglobulin G (IgG) protein. This receptor transports maternal IgG across the placenta or intestine to passively immunize fetal and newborn mammals against antigens to which the mother has been exposed. Björkman was surprised to find that the MHC groove of FcRn is collapsed and is not the binding site for Fc. In 1994, she solved the structure of FcRn with and without its immunoglobulin cargo. Using electron microscopy and confocal imaging, her lab is now determining how FcRn-immunoglobulin complexes move through polarized epithelial cells. "We are now imaging receptors in their native environment, that is, inside cells, in order to determine the pathways by which receptors transport ligands across epithelial cell barriers," she says.
Intrigued by the evolutionary origin of MHC proteins versus MHC homologs with nonimmune functions, Björkman identified FcRn's functional counterpart in chickens, FcRY, which transports immunoglobulins from hens to eggs. This protein turned out to have a totally different structure from mammalian FcRn or MHC proteins, without a hint of a groove.
Björkman has found several examples of proteins that share amino acid sequences and 3-D conformations with MHC proteins but do not have immune functions. One is the hemochromatosis (HFE) protein, which can lead to iron overload when mutated. The crystal structure of HFE, published by Björkman's group in 1998, showed that HFE also has only a vestigial groove and that its receptor binds elsewhere on the protein. "When the gene for HFE was discovered, you would have said it was for presenting something to the immune system," Björkman says. "And that would be totally wrong."
Another MHC-like molecule, M10, has an even more exotic function: binding possible pheromone receptors in a roof-of-the-mouth organ that influences mate selection in rodents. This protein has a groove, Björkman's group revealed in 2005, but the groove is surprisingly empty in the purified protein, suggesting that it normally holds something other than a peptide.
Other renegade MHC homologs that Björkman has studied include ZAG (Zn-α2-glycoprotein), which promotes fat loss and therefore wasting in cancer and AIDS patients. Like M10, ZAG has a groove that holds a nonpeptide. "So you can't take anything for granted," Björkman says. "If you find a sequence in the database and it has homology to something that has a known function, you can't necessarily believe that you have found that function."
Björkman's studies of viral proteins that evade immune surveillance by mimicking MHCs led to one of her current projects: trying to make antibody-like reagents against HIV. She also intends to do more imaging of live cells. "We have a couple of systems where we have gone from the crystallography of individual molecules or complexes to imaging receptors in cells," she says. "Now, we would like to do that with all the structural systems we have studied."
