Huntington’s disease (HD) is a progressive neurological disorder caused by a mutation in the HTT gene which encodes for the huntingtin protein. The current understanding of the function of HTT is limited, and there are only a few sources that indicate that it likely plays a critical role in several intracellular processes such as proper neural development. This gap in the scientific literature continues onto the mutant version of the protein. In patients with HD, we know that the mutant protein misfolds and forms aggregates. However, it is unclear if this is the direct cause for the symptoms of the disease or if it is a downstream symptom of some other problem. To obtain a more conclusive understanding of huntingtins role in the cell, the SGC is trying to obtain a structure for huntingtin protein.
So now the question is, how do we image such a molecule? At the SGC we use a technique called X-ray Crystallography. This is a process by which a very pure protein sample is crystallized and expose to X-rays. Using the diffraction of light through the crystal, we can piece together an atom by atom computer model of the sample protein. One complication with the huntingtin protein is that it contains several disordered regions which would likely prevent the formation of a consistent structure in the crystal. To get a full image of this protein, we need to find a way to force a consistent structure onto the disordered regions. Binding a protein to the disordered region causes consistent reordering of this region. The use of a BioID experiment has identified several proteins that bind huntingtin. In effect, this experiment consists of arming the huntingtin protein with a spray can. When another protein comes within its vicinity, it is tagged by the huntingtin molecule. By looking at tagged proteins, several interaction partners of huntingtin were identified.
During my time at the SGC, I plan to go through this list of tentative interaction partners, producing and purifying them for use in future experiments. Furthermore, this could allow us to obtain crystal structures of huntingtin in complex with an interaction partner. Obtaining a list of all the interaction partners of any given protein can give us a good idea of the function of the protein. This is because we can then attribute the interaction partners function to that of the protein of interest. In the coming weeks, I will continue to post updates on the progress of the purification of the hits from the BioID experiment.