Picture This: NMR provides a glimpse of amyloid fibrils (Amyloid; Structural biology)
Authors: Davenport R. John 
Published: August 2003

Protein clumps called amyloid fibrils crop up in numerous neurodegenerative diseases. But their structural details--which could reveal how they form and why they're linked to trouble--remain fuzzy. Now, two independent reports help bring amyloid fibrils into focus.

Although proteins that compose amyloid fibrils vary in amino acid sequence, the fibrils assume similar shapes: They all contain extensive β strands, peptide ribbons that lock together through hydrogen bonding. Because they are big, fibrils have escaped high-resolution scrutiny. In the new work, both groups use chemical tricks and nuclear magnetic resonance (NMR)--a technique that probes the chemical environment of atoms in a molecule--to follow the twists and turns of β
2
-microglobulin, a blood protein that deposits amyloid in kidney dialysis patients.

Putting many amyloid proteins--including β
2
-microglobulin--at low pH accelerates fibril growth, perhaps by shifting the protein partway toward the solid state. To determine the shape of this intermediate form, McParland and colleagues used NMR to assess β
2
-microglobulin's floppiness. At low pH, its ends unfolded readily, but its core remained in β strands. This β
2
-microglobulin intermediate resembles the intermediate form of transthyretin, an unrelated fibril-forming protein, says chemist and structural biologist Jeffrey Kelly of the Scripps Research Institute in La Jolla, California. The results hint that different amyloid proteins undergo similar structural changes as they make fibrils and support the idea that amyloid proteins unfold partially before congregating. What's still unclear, says Kelly, is whether the unfolded or the folded part is crucial for assembly.

In another study, Hoshino and colleagues probed the shape of β
2
-microglobulin fibrils. They plopped fibrils into heavy water. Deuterium atoms replace accessible hydrogen atoms in the protein but can't switch with hydrogen atoms tied up in bonds between amino acids. Then the researchers used dimethyl sulfoxide to stop the exchange of hydrogen and deuterium and dissolve the aggregates into unfolded proteins that are small enough to examine by NMR. The two-step process--first swapping accessible hydrogens in the aggregated state with deuterium and then analyzing the dispersed proteins to find the sites of exchange--is a "clever way" to study a structure that is too large for NMR, says Kelly. In the fibril, β
2
-microglobulin's ends open up, whereas normally free loops in the core regions become protected, perhaps by forming β strands. The researchers conclude that each protein in the amyloid fibril adopts a more extensive network of β strands than it does in its soluble condition, lending fibrils their rigidity and resistance to aggregate-busting enzymes.

The two studies together suggest that pliant ends and rigid centers allow proteins to cluster into amyloid fibrils. Whether this new structural information will generally apply awaits further study. "We need to look at a large number of the other [amyloid-forming] proteins," says Kelly. Mounting evidence suggests that small clumps that arise early in the aggregation process trigger disease. Researchers hope that understanding the shape of proteins in amyloid fibrils--and how they morph to get there--will help uncover ways to prevent the vexing coalescence.

--R. John Davenport

M. Hoshino, H. Katou, Y. Hagihara, K. Hasegawa, H. Naiki, Y. Goto, Mapping the core of the β
2
-microglobulin amyloid fibril by H/D exchange.
Nat. Struct. Biol.,
22 April 2002 [e-pub ahead of print].
[Abstract]
[Full Text]

V. J. McParland, A. P. Kalverda, S. W. Homans, S. E. Radford, Structural properties of an amyloid precursor of β
2
-microglobulin.
Nat. Struct. Biol.,
22 April 2002 [e-pub ahead of print].
[Abstract]
[Full Text]