The Melanocytic Protein Melan‐A/MART‐1 Has a Subcellular Localization Distinct from Typical Melanosomal Proteins

To delineate the role of the melanocyte lineage‐specific protein Melan‐A/MART‐1 in melanogenic functions, a set of biochemical and microscopical studies was performed. Biochemical analysis revealed that Melan‐A/MART‐1 is post‐translationally acylated and undergoes a rapid turnover in a pigmented melanoma cell line. Immunofluorescence and immunoelectron microscopy analyses indicated that Melan‐A/MART‐1 is mainly located in the Golgi area and only partially colocalizes with melanosomal proteins. Quantitative immunoelectron microscopy showed that the highest proportion of the cellular content of Melan‐A/MART‐1 was found in small vesicles and tubules throughout the cell, whereas the concentration was maximal in the Golgi region, particularly the trans‐Golgi network. Substantial labeling was also present on melanosomes, endosomes, ER, nuclear envelope, and plasma membrane. In early endosomes, Melan‐A was enriched in areas of the limiting membrane covered by a bi‐layered coat, a structural characteristic of melanosomal precursor compartments. Upon melanosome maturation, Melan‐A concentration decreased and its predominant localization shifted from the limiting membrane to internal vesicle membranes. In conjunction with its acylation, the high expression levels of Melan‐A in the trans‐Golgi network, in dispersed vesicles, and on the limiting membrane of premelanosomes indicate that the protein may play a role during the early stage of melanosome biogenesis.