Identification of a subunit interface in transthyretin amyloid fibrils: Evidence for self-assembly from oligomeric building blocks

Amyloid and prion diseases appear to stem from the conversion of normally f olded proteins into insoluble, fiber-like assemblies. Despite numerous stru ctural studies, a detailed molecular characterization of amyloid fibrils re mains elusive. In particular, models of amyloid fibrils proposed thus far h ave not adequately defined the constituent protein subunit interactions. To further our understanding of amyloid structure, we employed thiol-specific cross-linking and site-directed spin labeling to identify specific protein -protein associations in transthyretin (TTR) amyloid fibrils. We find that certain cysteine mutants of TTR, when dimerized by chemical cross-linkers, still form fibers under typical in vitro fibrillogenic conditions. In addit ion, site-directed spin labeling of many residues at the natural dimer inte rface reveals that their spatial proximity is preserved in the fibrillar st ate even in the absence of cross-linking constraints. Here, we present the first view of a subunit interface in TTR fibers and show that it is very si milar to one of the natural dimeric interchain associations evident in the structure of soluble TTR. The results clarify varied models of amyloidogene sis by demonstrating that transthyretin amyloid fibrils may assemble from o ligomeric protein building blocks rather than structurally rearranged monom ers.