Dual loss of ER export and endocytic signals with altered melanosome morphology in the silver mutation of Pmel17.

Pmel17 is a pigment cell-specific integral membrane protein that participates in the formation of the intralumenal fibrils upon which melanins are deposited in melanosomes. The Pmel17 cytoplasmic domain is truncated by the mouse silver mutation, which is associated with coat hypopigmentation in certain strain backgrounds. Here, we show that the truncation interferes with at least two steps in Pmel17 intracellular transport, resulting in defects in melanosome biogenesis.

A lumenal domain-dependent pathway for sorting to intralumenal vesicles of multivesicular endosomes involved in organelle morphogenesis.

Cargo partitioning into intralumenal vesicles (ILVs) of multivesicular endosomes underlies such cellular processes as receptor downregulation, viral budding, and biogenesis of lysosome-related organelles such as melanosomes. We show that the melanosomal protein Pmel17 is sorted into ILVs by a mechanism that is dependent upon lumenal determinants and conserved in non-pigment cells. Pmel17 targeting to ILVs does not require its native cytoplasmic domain or cytoplasmic residues targeted by ubiquitylation and, unlike sorting of ubiquitylated cargo, is insensitive to functional inhibition of Hrs and ESCRT complexes.

A novel splice variant of Pmel17 expressed by human melanocytes and melanoma cells lacking some of the internal repeats.

Pmel17 is a approximately 100 kDa pigment cell specific glycoprotein that plays a crucial part in the morphogenesis of melanosome precursors. Anti-Pmel17 immunoprecipitates from metabolically pulse labeled melanoma cells and melanocytes contain, in addition to full-length Pmel17, a glycoprotein that migrates with a lower relative molecular weight. Here we show that this glycoprotein is encoded by an mRNA that results from alternative splicing of the human Pmel17 gene from which a cryptic intron is excised.

Proprotein convertase cleavage liberates a fibrillogenic fragment of a resident glycoprotein to initiate melanosome biogenesis.

Lysosome-related organelles are cell type-specific intracellular compartments with distinct morphologies and functions. The molecular mechanisms governing the formation of their unique structural features are not known. Melanosomes and their precursors are lysosome-related organelles that are characterized morphologically by intralumenal fibrous striations upon which melanins are polymerized.