Unpack the structure of a protein complex and you can see how it might break apart and become inactive, resulting in potentially adverse consequences. For example, when the protein complex known as protein phosphatase 2A (PP2A) breaks apart, it may fail to deactivate pro-growth proteins and thereby contribute to cancer or neurodegenerative disease.
Keeping PP2A intact, or reactivating PP2A that has become inactivated, would seem a good way to preserve health. But how? To help answer that question, scientists based at the University of Wisconsin–Madison decided to figure out how the usually rugged PP2A complex might be vulnerable to disassembly-related inactivation.
The scientists, led by Yongna Xing, Ph.D., an associate professor of oncology with the UW Carbone Cancer Center and McArdle Laboratory for Cancer Research, found that PP2A disassembly is greatly enhanced by a butterfly-shaped protein called target of rapamycin (TOR) signaling pathway regulator (TIRPL). Also, the scientists learned how TIRPL-enhanced disassembly is more likely to occur when certain mutations in PP2A occur.
Details of this work appeared December 22 in the journal Nature Communications, in an article entitled “Methylation-Regulated Decommissioning of Multimeric PP2A Complexes.” This article extends previous work, also conducted by Dr. Xing’s team, that showed how inactivated PP2A has a regulatory protein, α4, attached. The new work explains how challenge with α4 is not sufficient to inactivate PP2A that is already active.
TIPRL “makes highly integrative multibranching contacts with the PP2A catalytic subunit, selective for the unmethylated tail and perturbing/inactivating the phosphatase active site,” the authors of the current article wrote. “TIPRL also makes unusual wobble contacts with the scaffold subunit, allowing TIPRL, but not the overlapping regulatory subunits, to tolerate disease-associated PP2A mutations, resulting in reduced holoenzyme assembly and enhanced inactivation of mutant PP2A.”
When the scientists challenged active PP2A complexes with both α4 and TIPRL, the complexes broke apart. Next, they determined the 3D structure of TIRPL with PP2A using X-ray crystallography.
“The structure shows how TIPRL can attack active PP2A complexes, even though it has a much lower affinity than the specificity subunits do for PP2A core,” Dr. Xing explained. “With the structure, we were able to identify how TIRPL can attack the complex, change its conformation, and, together with α4, make it fall apart robustly. It was hard to picture how this process could happen without structural insights.”
“Strikingly, TIPRL and the latency chaperone, α4, coordinate to disassemble active holoenzymes into latent PP2A, strictly controlled by methylation,” the authors of the Nature Communications article detailed. “Our study reveals a mechanism for methylation-responsive inactivation and holoenzyme disassembly, illustrating the complexity of regulation/signaling, dynamic complex disassembly, and disease mutations in cancer and intellectual disability.”
If we think of PP2A as a power screwdriver, the findings make a lot of practical sense. The core protein is the motorized base, and the specificity proteins—the ones that mix and match to help PP2A find the right target—are the screw heads. When you want to switch from a Phillips-head to a flathead screwdriver, you don't throw away the whole power screwdriver complex and buy a new one; rather you detach one screw head and attach another. Similarly, it is energy costly for a cell to degrade the entire PP2A complex, so TIPRL's role is to detach the specificity protein and recycle the PP2A core.
One of the more interesting findings from the structure was how flexible TIRPL is compared to the specificity subunits, prompting the researchers to ask how PP2A mutations commonly seen in cancer patients affect TIPRL binding. Using either a normal or a PP2A core containing these mutations, they measured how well TIPRL and the specificity subunits can bind to it. They found that the core mutations have almost no effect on TIPRL binding, but they drastically weaken the binding of specificity proteins. These mutations, then, likely cause a shift from active PP2A complexes to the disassembled and inactive form.
“In many diseases, including cancers and neurodegenerative diseases, PP2A, in general, is less active, often due to mutations,” Dr. Xing noted. “This structure helps explain how those mutations lead to downregulation of PP2A by shifting the balance toward TIPRL-induced complex dissociation.”
With the structure in hand, Dr. Xing expects to be able to better understand the cycle of PP2A activation and inactivation, and how it regulates cell growth.
“For example, active PP2A is known to inhibit K-ras, a protein that drives growth in many tumors and currently has no good clinical inhibitors,” Dr. Xing said. “If you can find a way to reactivate PP2A, it could be very important in treating those cancers.”