Amyloids, whether of the amyloid-β of Alzheimer disease, or of other peptides such as transthyretin, are composed of long polymeric fibrils that stick together like glue. But how are the fibers formed? Does a chain grow by the addition of single molecules at the end, or are medium-length pieces, or protofibrils, of up to 10, 20, or more monomers shunted together like a freight train? The answer is important because evidence suggests that protofibrils are the most toxic form of Aβ (see ARF related news story), and understanding their formation and consumption is tantamount to understanding the dynamics of Aβ toxicity.
In the October PLoS Biology, already available online, Jonathan Weissman and colleagues from the University of California at San Francisco report that amyloid derived from the yeast prion, Sup35, forms by the simple addition of monomers to the end of a growing chain, even in the presence of oligomers. The finding suggests that other amyloids may grow in a similar fashion.
First author Sean Collins and colleagues used the NM domain of Sup35 to measure amyloid formation in an analytical ultracentrifuge, which revealed that NM existed predominantly as monomers during the process. Using a thioflavin binding assay, they were also able to show that the kinetics of amyloid formation fit a monomeric addition model perfectly. Seeding the reaction with preformed fibers also resulted in chain growth by monomer addition. But perhaps the strongest evidence for monomer addition came from direct visible measurement of fiber growth. Using a total internal reflection fluorescence microscope, Collins and colleagues were able to measure the addition of fluorescently labeled NM to an NM fiber bound to a glass slide. The fluorescence accumulated specifically at the fiber ends, and those spots were stable—most of them could be seen 15 seconds later, whereas less than 20 percent of spots survived as long if fiber ends were absent—suggesting that the monomers had bound tightly.
“Many kinetic features seen for NM polymerization are shared by other amyloidogenic proteins, suggesting that monomer addition may represent a mechanism of amyloid growth common to other fibers,” state the authors. They also speculate that if this is true, then agents designed to promote direct fiber formation could help prevent the formation of the shorter, potentially toxic oligomers.—Tom Fagan