The most pathogenic transthyretin variant, L55P, forms amyloid fibrils under acidic conditions and protofilaments under physiological conditions

The L55P transthyretin (TTR) familial amyloid polyneuropathy-associated var iant is distinct from the other TTR variants studied to date and the wild-t ype protein in that the L55P tetramer can dissociate to the monomeric amylo idogenic intermediate and form fibril precursors under physiological condit ions (pH 7.0, 37 degrees C). The activation barrier associated with L55P-TT R tetramer dissociation is lower than the barrier for wild-type transthyret in dissociation, which does not form fibrils under physiological conditions . The L55P-TTR tetramer is also very sensitive to acidic conditions, readil y dissociating to form the monomeric amyloidogenic intermediate between pH 5.5-5.0 where the wild-type TTR adopts a nonamyloidogenic tetrameric struct ure. The formation of the L55P monomeric amyloidogenic intermediate involve s subtle tertiary structural changes within the beta-sheet rich subunit as discerned from Trp fluorescence, circular dichroism analysis, and ANS bindi ng studies. The assembly of the L55P-TTR amyloidogenic intermediate at phys iological pH (pH 7.5) affords protofilaments that elongate with time. TEM s tudies suggest that the entropic barrier associated with filament assembly (amyloid fibril formation) is high in vitro, amyloid being defined by the l aterally assembled four filament structure observed by Blake upon isolation of "fibrils" from the eye of a FAP patient. The L55P-TTR protofilaments fo rmed in vitro bind Congo red and thioflavin T (albeit more weakly than the fibrils produced at acidic pH), suggesting that the structure observed prob ably represents an amyloid precursor. The structural continuum from misfold ed monomer through protofilaments, filaments, and ultimately fibrils must b e considered as a possible source of pathology associated with these diseas es.