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A melanocyte is a pigment-producing biological cell that synthesizes melanin in specialized compartments called melanosomes. In skin, melanosomes are transferred to neighbouring keratinocytes, so the site of pigment production differs from much of its final distribution. Melanin helps attenuate ultraviolet radiation; differences in pigmentation depend on pigment production and handling, rather than simply the number of melanocytes. ^[ref]

A melanosome is a specialized, membrane-bounded organelle in which melanin is synthesized and stored. Melanocytes produce melanosomes; pigment can subsequently be transferred to neighbouring keratinocytes. The melanosome is the compartment, whereas melanin is its pigment cargo. Maturation involves changes in internal organization as well as accumulation of pigment. ^[ref]

An experiment connected that organization to a particular protein. Expression of Pmel17 in nonpigmented cells produced internal striations resembling those of immature melanosomes, within multivesicular compartments. The result supported a structural role for Pmel17 in constructing a scaffold before extensive pigment deposition. It did not show that expression of one protein produces every function of a mature melanosome. ^[ref]

Pmel17 is a pigment-cell glycoprotein that helps build the internal scaffold of developing melanosomes, the organelles in which melanin is produced and stored. It is synthesized as a membrane protein, but processing produces a large lumen-facing component associated with the characteristic internal striations of immature melanosomes. The scaffold and the pigment deposited on it are different materials.

Joanne F. Berson and colleagues expressed Pmel17 in nonpigmented cells. Electron microscopy revealed striation-like structures inside multivesicular bodies, compartments that normally lack the specialized pigment-cell architecture. Together with localization and processing experiments in pigmented cells, this supported a direct role for Pmel17 in constructing the melanosome interior. It demonstrates how adding a particular protein can reshape a general intracellular compartment into a specialized organelle.^[ref]

A premelanosome is an immature melanosome whose internal architecture is developing before the compartment becomes heavily filled with melanin. In a characteristic maturation sequence, an initially vesicular compartment develops internal striations and then accumulates pigment on the resulting scaffold. “Immature” therefore describes a stage of an organelle's development, rather than a different kind of pigment.

Pmel17 helps construct that scaffold. Joanne F. Berson and colleagues localized processed Pmel17 to internal vesicles and striations of premelanosomes. Expressing the protein in nonpigmented cells also generated striation-like structures in multivesicular bodies.^[ref] Building the scaffold can thus be distinguished experimentally from synthesizing the pigment that later occupies it.