Amyloid fibrils from the mammalian protein prothymosin α

Mammalian prothymosin a, a small (12 kDa) and extremely acidic protein (pI 3.5), is a member of the growing family of `natively' unfolded proteins. We demonstrate that at low pH ( similar to 3) and high concentrations, prothymosin a is capable of forming regular elongated fibrils with flat ribbon structure 4-5 nm in height and 12-13 nm in width as judged from scanning force and electron microscopy. These aggregates induced a characteristic spectral shift of thioflavin T fluorescence and their circular dichroism spectra were indicative of significant beta-sheet content, suggesting formation of classical amyloid. Our findings indicate that natively unfolded proteins may have a general propensity to form amyloid fibrils under conditions inducing partially folded conformations. (C) 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.