Involvement of Dynein and Spectrin with Early Melanosome Transport and Melanosomal Protein Trafficking

Abstract

Melanosomes are unique membrane-bound organelles specialized for the synthesis and distribution of melanin. Mechanisms involved in the trafficking of proteins to melanosomes and in the transport of mature pigmented melanosomes to the dendrites of melanocytic cells are being characterized, but details about those processes during early stages of melanosome maturation are not well understood. Early melanosomes must remain in the perinuclear area until critical components are assembled. In this study, we characterized the processing of two distinct melanosomal proteins, tyrosinase (TYR) and Pmel17, to elucidate protein processing in early or late steps of the secretory pathway, respectively, and to determine mechanisms underlying the subcellular localization and transport of early melanosomes. We used immunological, biochemical, and molecular approaches to demonstrate that the movement of early melanosomes in the perinuclear area depends primarily on microtubules but not on actin filaments. In contrast, the trafficking of TYR and Pmel17 depends on cytoplasmic dynein and its interaction with the spectrin/ankyrin system, which is involved with the sorting of cargo from the plasma membrane. These results provide important clues toward understanding the processes involved with early events in melanosome formation and transport.

The Regulation of Skin Pigmentation

“…Intracellular pH is an important consideration to the regulation of TYR function (46), not only because intramelanosomal pH dramatically affects catalytic functions but also because a correct pH gradient is critical to the sorting pathway responsible for delivery of melanosomal proteins (47). There are many instances where normal levels of functional TYR are produced by melanocytic cells and yet little or no melanin is formed; it is even thought that intracellular pH plays an important role in regulating pigment production in various types of skin according to racial/ ethnic origin (48).…”

βIII Spectrin Regulates the Structural Integrity and the Secretory Protein Transport of the Golgi Complex

“…Post-Golgi trafficking of glutamate transporter EAAT4 and other synaptic proteins is also impaired when mutations in the ␤III spectrin occur in spinocerebellar ataxia type 5 (23)(24)(25)(26)(27). ␤ spectrin participates in the dynein-mediated cargo (melanin) and organelle transport (melanosomes) along microtubules (28), and ␤III spectrin binds to the dynactin subunit Arp1 in vitro (29). In late endosomes, it requires the active action of the tripartite complex Rab7-RILP and ORP1L for their dynein-dependent translocation to the minus-end of microtubules (30).…”

Heterogeneity of neural crest-derived melanocytes

“…These melanosomes are spherical vacuoles with no internal structural components. In stage II, the visible fibrillar matrix formed by glycoproteins (Pmel17, MART-1), melanosomes elongates, tyrosinase is present [71,77,79]. In stage III, melanosomes begin melanin production that polymerizes and settle on the internal fibrills net.…”

“…They are often separated into "early" (Stage I and II) and "late" melanosomes (Stage II and IV) according to lack or presence of pigment [50,58,76,77].…”