Structural Snapshots of Tankyrase, a Protein Involved in a Rare Genetic Disorder and Potential Cancer Target
DECEMBER 15, 2011
A discovery made with the help of x-rays from the Advanced Photon Source (APS) at Argonne National Laboratory provides researchers with a greater understanding of the protein Tankyrase, which is linked to the bone development disorder cherubism and involved in a myriad of cellular processes, and may also lead to the development of new designer drugs to treat cancer.
Researchers from the Samuel Lunenfeld Research Institute (SLRI), the Ontario Cancer Institute, the University of Toronto, and Saint Michael’s Hospital have gained the first structural insight into precisely how the Tankyrase enzyme correctly identifies its targets, or substrates.
“Until now, we did not understand, from a structural perspective, how Tankyrase identifies its substrates,” said Frank Sicheri, Lunenfeld Senior Investigator and one of the lead authors of the study. “At atomic resolution, we now have a clearer picture of what these substrates may be, and have new insight into possible novel functions of Tankyrase.”
The team’s findings are published in the December 9 issue of the leading biomedical journal Cell.
Tankyrase is a poly(ADP-ribose)polymerase (PARP)—one protein of a family of enzymes that modify other proteins with chains of ADP-ribose and affect many cellular processes. The modification reactions carried out by Tankyrase can directly alter some proteins’ functions, bring proteins together in protein complexes, or can mark others for degradation.
Initially intrigued by Tankyrase because of its involvement in cherubism (a rare genetic disorder caused by mutations in the signaling protein 3BP2), the researchers built upon the findings of Robert Rottapel’s laboratory at the Ontario Cancer Institute. This laboratory found that Tankyrase normally recognizes 3BP2 and targets it for destruction. The amino acids mutated in cherubism coincide with precisely the region in 3BP2 that is recognized by Tankyrase, or the “Tankyrase binding motif.” Cherubism mutations in 3BP2 prevent binding of Tankyrase and therefore result in the accumulation of 3BP2 protein in the cell. Rottapel’s findings also appear in the same issue of Cell.
Cherubism, which first manifests in early childhood, is characterized by abnormal bone tissue in the lower part of the face. The lower jaw and the upper jaw become enlarged as bone is replaced with painless, cyst-like growths. These growths give the cheeks a swollen, rounded appearance and often interfere with normal tooth development. In some people the condition can cause problems with vision, breathing, speech, and swallowing. Enlargement of the jaw usually continues throughout childhood and stabilizes during puberty. It can be gradually replaced with normal bone in early adulthood.
The goal of Sicheri and his team was to uncover the exact mechanism by which Tankyrase recruits its substrates, to explain why cherubism mutations in 3BP2 disrupt Tankyrase binding and thereby learn more about how the enzyme works.
The researchers utilized x-ray crystallography for their experiments at the Northeastern Collaborative Access Team x-ray beamlines 24-ID-C and 24-ID-E at the U.S. Department of Energy Office of Science’s APS. Their studies led them to a determination of the structures of the portion of Tankyrase responsible for substrate binding, bound to a range of different substrates including 3BP2. Using the technique fluorescence polarization, the researchers then determined the essential signature of the Tankyrase binding motif by which Tankyrase identifies its substrates.
The researchers scanned the entire inventory of human proteins, searching for the signature sequence that is recognized by Tankyrase, correctly predicting many possible new substrates for the enzyme. The result: a deeper understanding of the biology behind Tankyrase’s cellular activities.
“Our work provides answers to two big questions. Firstly, we obtained a visual snapshot of how Tankyrase recognizes its substrates and how mutations characteristic of cherubism lead to illness,” said Sebastian Guettler, a post-doctoral Fellow in Sicheri’s and Tony Pawson’s labs at SLRI and first author of the study. “Secondly, we learned more about the possible cellular tasks performed by Tankyrase. The apparent abundance of potential Tankyrase targets and the variety of cellular functions they perform suggests that the complexity of Tankyrase’s biological functions has been underappreciated to date.”
Inhibitors of PARPs, and among them Tankyrase, have gained considerable attention recently as potential new anti-cancer agents. Inhibition of Tankyrase function may hold promise for treating certain breast cancers as well as other cancers, and therefore the present study may help refine treatment strategies for blocking Tankyrase.