The tetrameric protein transthyretin dissociates to a non-native monomer in solution - A novel model for amyloidogenesis

In amyloidosis, normally innocuous soluble proteins polymerize to form inso luble fibrils, Amyloid fibril formation and deposition have been associated with a wide range of diseases, including spongiform encephalopathies, Alzh eimer's disease, and familial amyloid polyneuropathies (FAP), In certain fo rms of FAP, the amyloid fibrils are mostly constituted by variants of trans thyretin (TTR), a homotetrameric plasma protein implicated in the transport of thyroxine and retinol, The most common amyloidogenic TTR variant is V30 M-TTR, and L55P-TTR is the variant associated with the most aggressive form of FAP, Recently, we reported that TTR dissociates to a monomeric species at pH 7.0 and nearly physiological ionic strengths (Quintas, A, Saraiva, M, J,, and Brito, R, M, (1997) PEES Lett, 418, 297-300). Here, we show that t he tetramer dissociation is apparently irreversible; and based on intrinsic tryptophan fluorescence and fluorescence quenching experiments, we show th at the monomeric species formed upon tetramer dissociation is non-native. W e also show, based on 1-anilino-8-naph-thalenesulfonate binding studies, th at this monomeric species appears not to behave like a molten globule, Thes e data allowed us to propose a model for TTR amyloidogenesis based on tetra mer dissociation occurring naturally under commonly observed physiological solution conditions.