Fig. 7.

sRPT aggregation and fibril maturation mirror melanosomal conditions. S represents soluble sRPT at pH 5, 6, or 7. F_I and F_M represent immature and mature sRPT fibrils, respectively. At pH 4, sRPT rapidly forms F_I, which converts into F_M at pH 5 and 6. This process mimics melanosome maturation, which is at ~pH 4 in stages I–II, and reaches near neutral pH upon full maturation (stage IV). sRPT displays concentration-dependent behavior at pH 5, forming F_I at concentrations above 30 µM and F_M at concentrations below 15 µM. At pH 6, sRPT forms F_M. Both F_I and F_M dissolve into their soluble form at pH 7.