Morph mystery

Some proteins may morph from one structure to another - in this case, a cube-like structure can switch to a tetrahedal shape.

SOURCE: Eileen Jaffe / Fox Chase Cancer Center

For years, Fox Chase Cancer Center researcher Eileen Jaffe's findings on the behavior of a new mutant enzyme have been met with disbelief by the protein community. When she presented at conferences, people told her "proteins just don't do" what she claims they do. When Jaffe suggested to a researcher after his conference presentation that he might be dealing with a similar phenomenon to the one she claims to see, he told her she was a troublemaker.

Jaffe first encountered the mutant enzyme in the British Journal of Haematology (106:931-7, 1999), which described the case of a Swedish girl born in 1996. The girl carried a novel mutation that produced an alternate version of an enzyme called porphobilinogen synthase (PBGS), vital in the heme biosynthesis pathway.

Jaffe had been studying PBGS for more than 10 years, intrigued by its high affinity for binding lead, and decided it would be interesting to study the new mutant version. That's when the trouble started. For one, the mutant protein ran drastically differently from the wild-type on both a native gel electrophoresis and an ion exchange column - two signs that the mutant had a significantly different structure from the wild-type. Jaffe was surprised, since the mutation led to only a single amino acid substitution. "We said 'Boy this is weird'," she recalls.

Jaffe knew that the wild-type protein had an octameric quaternary structure, meaning that its active form contained eight subunits. Proteins can change shape when the subunits shift in relation to each other, but they traditionally keep the same number of subunits.

In 2003, Jaffe sent the mutant protein to a crystallographer at Haverford College. He told her the protein seemed to be hexameric, only containing six subunits. "I said 'It can't be a hexamer, we know it's an octamer'," Jaffe remembers, holding fast to the traditional idea that proteins can change conformations but maintain their quaternary structure. But when the crystal structure was complete, there was no denying it: the mutant protein was a hexamer.

Using mass spectrometry to examine both protein structures in Escherichia coli cells at the same time, Jaffe saw that, if she added the correct substrate, the two proteins would interconvert - the hexamer could go to the octamer and vice versa - so even the wild-type was capable of switching. "The [protein] comes apart into disassociated parts, changes shape, and comes back together," says Jaffe, standing in the hallway outside her lab, surrounded by drawings of pink and blue bubbly clumps, meant to illustrate the two protein structures. Her group called the shape-shifting proteins morpheeins.

Jaffe has shown in plant PBGS that a small molecule can lock the mutant form in place and prevent it from becoming active. Based on various protein behaviors in assays, Jaffe surmises that dozens of other proteins like HIV integrase and cancer inhibitor p53 might be morpheeins themselves, suggesting researchers could design small molecules to keep these proteins inactive.

"A lot of lines of investigation depend on each protein having one structure," says Jaffe. "For example, people who use tags to purify proteins - they are going to miss any morpheeins because they're purifying on the basis of a tag," which pulls out all proteins with the tag, not certain structural versions of the protein.

"These morpheeins are most likely not specific to PBGS," agrees Patrick Loria, professor of chemistry at Yale University, in an E-mail. "There are suggestions throughout the literature that this phenomena may be more widespread," such as with tumor necrosis factor (TNFa) (Science, 310:1022-5, 2005) and HIV integrase (PNAS, 104:8316-21, 2007). Even so, he adds, PBGS remains the only well-documented case (Trends Biochem Sci, 30:490-7, 2005). Still, Jaffe faces much skepticism. "Nobody's ever seen this before. They've set the evidence bar really high, and that's a good thing."