By using high-pressure freezing of MNT-1 melanoma cells and freeze substitution to optimize ultrastructural preservation followed by double tilt 3D electron tomography, we show that the amyloid-like fibrils begin to form in multivesicular compartments, where they radiate from the luminal side of intralumenal membrane vesicles. Before fibril formation Pmel17 traffics through multivesicular endosomal compartments, but how these compartments participate in downstream events leading to fibril formation is not fully known. The integral membrane protein Pmel17 is a component of the fibrils, can nucleate fibril formation in the absence of other pigment cell-specific proteins, and forms amyloid-like fibrils in vitro. Early stage melanosomes are characterized morphologically by intralumenal fibrils upon which melanins are deposited in later stages. Melanosomes are lysosome-related organelles (LROs) in which melanins are synthesized and stored.
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