Regulation of the distribution of carotenoid droplets in goldfish xanthophores and possible implication to secretory processes

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Abstract
In goldfish xanthophores, the formation of pigment aggregate requires: (1) that a pigment organelle (carotenoid droplet) protein p57 be in the unphosphorylated state; (2) that self‐association of pigment organelles occur in a microtubule‐independent manner; and (3) that pigment organelles via p57 associate with microtubules. In the fully aggregated state, the pigment organelles are completely stationary. Pigment dispersion is initiated by activation of a cAMP‐dependent protein kinase, which phosphorylates p57 and allows pigment dispersion via an active process dependent on F‐actin and a cytosolic factor. This factor is not an ATPase, and its function is unknown. However, its abundance in different tissues parallels secretory activity of the tissues, suggesting a similarity between secretion and pigment dispersion in xanthophores. The identity of the motor for pigment dispersion is unclear. Experimental results show that pigment organelles isolated from cells with dispersed pigment have associated actin and ATPase activity comparable to myosin ATPase. This ATPase is probably an organelle protein of relative molecular mass ∼72,000, and unlikely to be an ion pump. Isolated pigment organelles without associated actin have 5× lower ATPase activity. Whether this organelle ATPase is the motor for pigment dispersion is under investigation. The process of pigment aggregation is poorly understood, with conflicting results for and against the involvement of intermediate filaments.