For 25 years, it has been known that mutation of the protein huntingtin (HTT) causes the neurodegenerative disorder, Huntington’s disease. The disease-causing variant contains a much longer repeat of glutamine residues at the N-terminus. However, much remains unknown about the protein’s structure, clouding our understanding of its true function. Fortunately, new cryo-electron microscopy work finally reveals the three-dimensional structure of HTT. These findings, published in Nature, come from a team led by researchers from Max Planck Institute of Biochemistry in Martinsried and Ulm University.
Fernández-Busnadiego mentions the hurdles the team has faced in solving the structure. "First of all, cryo-electron microscopy has only been optimized in recent years to look at protein structures with almost molecular resolution. The second reason is that the huntingtin protein is very flexible in its structure. Just now, we have found also a solution for this problem."
The solution was to stabilize HTT enough in a given conformation so that the images taken will be clear enough. The team had thus been looking for proteins that interact with HTT well enough to stabilize it. What they finally found was HAP40 (HTT-associated protein 40; encoded by three F8A genes in humans). HAP40 binds HTT in a cleft and contacts three structural domains by hydrophobic and electrostatic interactions, providing stability.
"Huntingtin in connection with HAP40 is stabilized in a particular conformation. Thus, averaged over many pictures, we were able to derive the three-dimensional structure," said senior co-author Stefan Kochanek. The team reports the solved structure of the full-length human HTT at an overall resolution of 4 Å, being largely alpha-helical and consisting of three major domains.
"Now that we know the exact structure of huntingtin, we can further study which areas of huntingtin are particularly important and how other proteins cooperate with huntingtin functionally. In this way structures could be deduced to be targeted therapeutically by certain drugs," says Kochanek.
The team’s paper concludes that these findings “pave the way for improved understanding of the diverse cellular functions of HTT.” Other benefits include ongoing drug development for Huntington’s disease, such as with gene-silencing antisense nucleotides aimed at decreasing mutant HTT. Better understanding of the protein structure may allow for better drug specificity toward the disease-causing variant.
Image: The 3D molecular structure of the human huntingtin-HAP40 protein complex. Image courtesy of the Protein Data Bank.