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A glycoprotein is a protein carrying one or more covalently attached carbohydrate groups. Attachment occurs through glycosylation; the carbohydrate component is part of the modified molecule, rather than simply dissolved beside it. Protein sequence alone therefore need not specify the complete chemical form measured in a biological preparation.

Pmel17 illustrates why the distinction is functional. It is a glycoprotein made as a membrane protein, then processed and organized inside developing premelanosomes. Experiments followed its molecular processing and its localization to internal vesicles and striations.^[ref] Calling it a glycoprotein identifies a chemical class; explaining its function still requires knowing its processing, position and interactions with other components.

Joanne F. Berson led a 2001 study of how Pmel17 helps form premelanosomes, the developing compartments in which melanin is deposited. The researchers followed processing and localization of the protein, then expressed it in nonpigmented cells. Expression alone initiated characteristic internal striations within multivesicular bodies.^[ref]

That reconstruction addressed causation more directly than finding Pmel17 beside existing pigment: a normally nonpigmented cell acquired part of the specialized internal architecture after the protein was introduced. It established a role in compartment construction without implying that one protein recreates every step of pigment production.

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.