The Role of Structural Dynamics in Determining the Prion Strain Diversity

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Prion proteins exhibit alternate structural states and are associated with a number of devastating transmissible diseases. Recent discoveries have revealed the emerging functional roles of prions in a wide range of organisms. For instance, the self-perpetuating conformational change coupled with amyloid formation of a yeast prion protein, Sup35p, a translational termination factor in yeast, is responsible for novel [PSI+] prion phenotypes in Saccharomyces cerevisiae. The 253-residue NM domain of Sup35p is an intrinsically disordered segment and is sufficient for [PSI+] prion initiation and propagation. The NM amyloid recapitulates one of the most spectacular phenomena of prions, namely, the strain-diversity. Earlier it has been shown that two well-defined strains of [PSI+] can be created in vitro. The molecular origin of these strains is postulated to involve diverse, yet related, conformational states and supramolecular packing of proteins within the amyloid fibrils. However, the precise structural and dynamical variations between the prion strains and their distinct physiological impacts remain elusive. To elucidate the structural origins of the prion strains, we took advantage of the fact that NM is devoid of tryptophan and created 19 single tryptophan mutants encompassing the entire length of NM. After establishing that these mutants behave similar to wild-type, we recorded a number of steady-state and dynamic fluorescence readouts that revealed the residue-specific dynamics and supramolecular packing within the amyloids responsible for different strains. The structural differences in two prion strains provided unique molecular insight into the differential binding of Hsp104 that is known to govern the strain propagation. The strain-diversity was further elucidated using time-resolved emission spectra that provided intriguing insights into the water relaxation dynamics within the amyloid architecture. Taken together, our results provide important biophysical clues in discerning the prion strain-diversity in a residue-specific manner.