Researchers reported a basic science finding that might someday lead to better treatments for neurodegenerative diseases like a hereditary form of Lou Gehrig's disease. Their work identifies details in how molecules bind to each other for transport into the nucleus of nerve cells in the brain and spinal cord. Proper binding and transport appear essential for the protein named FUS. The study was published in the journal Cell.

They identified a family of nuclear localization signals, dubbed the PY-NLS, used by the FUS protein. A PY-NLS sits at one end of the elongated FUS molecule and acts like a ZIP code for delivery of the FUS protein to the cell's nucleus. The NLS she identified became recognized as central to the development of fALS in which the FUS protein cargo cannot properly enter the nucleus and instead aggregates in the cell's cytoplasm surrounding the nucleus.

Protein Aggregation

Her discoveries allowed neuroscientists to figure out how FUS mutations lead to the aggregation of the FUS protein in the cytoplasm, but many questions remain unanswered. The mutant FUS forms little liquid droplets within 10 minutes, and these become less liquid and more solid over time, forming fibrils if the sample is left for 24 hours.

In studies of the purified FUS and Kapβ2 proteins, they found that the Kapβ2 blocks phase separation of FUS and that the importin's ability to block phase separation depends on its ability to read the ZIP code of the PY-NLS nuclear localization signal. When the ZIP code is unreadable, as in mutated FUS, phase separation into droplets continues unimpeded.

If you add a small amount of Kapβ2 protein to FUS proteins that have formed into droplets, within five minutes the droplets disappear. So the importin Kapβ2 is playing two roles: It is the transporter but also protects the FUS protein against phase separation and aggregation.

They found that under normal conditions, the Kapβ2 and FUS proteins bind very strongly to each other because of interactions at the nuclear localization signal, the ZIP code-like PY-NLS that sits at one end of the long FUS protein molecule. They also found that strong binding at the PY-NLS enables weaker and very brief interactions between other parts of the importin molecule and many other regions of the FUS protein.

X-ray Crystallography

The NMR equipment is highly sensitive to time intervals, making it possible to 'see' the transient binding of the two molecules. Doing so would be impossible with a static system such as X-ray crystallography. Binding at the right time, in the right place, in the right way is really important.

All this transient binding appears to keep the FUS protein from interacting with itself and inhibits assembly into droplets. Those droplets are probably the initial stages of the aggregates that are a hallmark of familial ALS. If we could find a way to keep the droplets from forming, perhaps we could change the course of that neurodegenerative disease.