Initiation and elongation in fibrillation of ALS-linked superoxide dismutase.

Familial amyotrophic lateral sclerosis (fALS) caused by mutations in copper-zinc superoxide dismutase (SOD1) is characterized by the presence of SOD1-rich inclusions in spinal cords. Similar inclusions observed in fALS transgenic mice have a fibrillar appearance suggestive of amyloid structure. Metal-free apo-SOD1 is a relatively stable protein and has been shown to form amyloid fibers in vitro only when it has been subjected to severely destabilizing conditions, such as low pH or reduction of its disulfide bonds. Here, by contrast, we show that a small amount of disulfide-reduced apo-SOD1 can rapidly initiate fibrillation of this exceptionally stable and highly structured protein under mild, physiologically accessible conditions, thus providing an unusual demonstration of a specific, physiologically relevant form of a protein acting as an initiating agent for the fibrillation of another form of the same protein. We also show that, once initiated, elongation can proceed via recruitment of either apo- or partially metallated disulfide-intact SOD1 and that the presence of copper, but not zinc, ions inhibits fibrillation. Our findings provide a rare glimpse into the specific changes in a protein that can lead to nucleation and into the ability of amyloid nuclei to recruit diverse forms of the same protein into fibrils.