Mechanisms of Functional Amyloid Formation

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Pmel17 fibrils serve as the structural scaffolding required for melanin deposition in human skin and eyes. Melanin is synthesized in melanosomes, organelles related to both endosomes and lysosomes, and stored in melanocytes, cells responsible for pigmentation. While the melanosome maturation process has been shown to involve four distinct stages that have been characterized in detail at the ultrastructural level by transmission electron microscopy (TEM), the molecular nature of the intralumenal Pmel17 fibrils during each of these stages is not known. Moreover, which polypeptide domain solely or partly constitutes the amyloid core of the Pmel17 filaments also remains to be defined. We have begun to study the repeat domain (RPT, residues 315-444), and essential luminal polypeptide region of Pmel17, as a model system of conformational change from soluble and unstructured monomer to aggregated, beta-sheet-containing fibrils. To mimic the changing acidic pH conditions of the maturing melanosome, we measured RPT amyloid formation kinetics as a function of solution pH. Since tryptophan emission is highly sensitive to solvent polarity, local conformational changes, and protein-protein interactions, we exploited the only intrinsic tryptophan (Trp423) as a site-specific fluorescent probe of amyloid structure and aggregation kinetics. We find that Trp423 is exquisitely sensitive to soluble and fibrillar RPT conformation with spectral properties (intensity and mean wavelength) exhibiting distinct temporal changes under the various solution conditions examined. Particularly, spectroscopic differences highlight distinct amyloid morphologies as visualized by TEM and thioflavin T activity. Furthermore, fibril formation kinetics are highly pH dependent and we identified a critical range of solution pH (4.5 to 5.5) for RPT aggregation suggesting that protonation of specific carboxylic acid side chains is critical in facilitating the structural rearrangement necessary for amyloid formation. Upon titration to neutral pH, these fibrils dissolve, supporting a regulatory mechanism whereby if released and exposed to neutral cytosolic pH, RPT would adopt a non-toxic, soluble form. Our current efforts are focused on understanding at the molecular level, which residue(s) contribute to the critical pH regime of RPT amyloid formation.