Fibre folding

In work that will aid scientists' understanding of amyloid fibres, best known as the plaque that gunks up neurons in people with neurodegenerative illnesses such as Alzheimer's, MIT and Whitehead Institute researchers have developed a technique to probe the fibres' architecture.

Even though amyloids are common and implicated in a host of conditions that also include Creutzfeldt-Jacob disease, the human analog of mad cow disease, researchers haven't been able to identify their precise molecular structures.

Conventional techniques used to image proteins, such as X-ray crystallography and nuclear magnetic resonance imaging, don't work with fibrous structures such as amyloids. And scientists depend on these high-resolution images of molecules in order to study their function.

Now, researchers led by MIT Professor of Biology and Whitehead Member Susan Lindquist have found a way to work around these limitations, illuminating the configuration of these sometimes pernicious molecules.

"These findings give us some fundamental insights in how amyloid fibres form," says Lindquist "They solve the important problem of identifying the intermolecular contacts that hold the amyloid fibre together."

Amyloid fibres are often composed of prions - proteins that misfold and recruit neighbouring proteins to misfold as well, a process that Lindquist calls a "conformational cascade."

When such a cascade occurs, the prions join and form amyloid fibres. (While not all amyloids are composed of prions, all known prions, in their transmissible states, form amyloid fibres.) But still, many scientists have been frustrated by their inability to gain anything more than a limited understanding of an amyloid's architecture.

Rajaraman Krishnan, a postdoctoral researcher in Lindquist's lab, found a way around that problem using strains of yeast. Rather than develop a single high-tech method for solving the amyloid structure, he instead used a combination of low-resolution tools to analyze varieties of prion strains and piece together the puzzle of how amyloids form.

Krishnan was able to identify the precise segment at which the prions interact -something that no one had done before him with a real prion. To do this, he took a variety of yeast prion strains and modified them in such a way that if particular designated regions came into contact with each other, they would emit a fluorescent signal, allowing him to map the pattern by which the different strains of prions interacted with each other.

He found that each prion molecule had only two points at which they connected to other prion molecules. One point he called the "head," the other the "tail." The head of one prion would only interact with the head of another prion, and likewise with tails. Remarkably, the same prion from the same yeast species could sometimes fold differently, and this fold would form its own cascade of interactions. In this altered form, the prion molecules interact in slightly different places, presenting different surfaces to promote the conversion of other prion molecules.

This research work, which will be published in the June 9 issue of the journal Nature, was supported by the DuPont-MIT Alliance and an NIH grant.