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

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 approximately 72,000, and unlikely to be an ion pump. Isolated pigment organelles without associated actin have 5x 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.

Characterization of a novel filament system in goldfish xanthophores

“…Pigment aggregation towards the cell center was shown to depend on the integrity of microtubules [Tchen et al, 1988;Chen and Wang, 1993] and dephosphorylation of a p57 protein on carotenoid pigment granules was shown to be coincident with aggregation [Lynch et al, 1986]. Intermediate filaments were identified in xanthophores and shown to be biochemically distinct from vimentin, consisting of four polypeptides of 60, 45, 56, and 51 kD [Walker et al, 1985].…”

“…In xanthophores, pigment dispersion apparently did not involve microtubules [Chen and Wang, 1993] while ATP, F-actin, and phosphorylation of a p57 protein of carotenoid droplets were proposed to be indispensable for this event [Lynch et al, 1986;Tchen et al, 1988;Yu et al, 1990]. Treatment with anti-actin antibody, phalloidin, or DNAase in digitonin-treated xanthophores completely inhibited pigment dispersion [Yu et al, 1990].…”

“…Xanthophores were chosen because the cells can be isolated in relative purity and large quantities [Lo et al, 1982], therefore providing a ready source of xanthophore-specific antigens. Moreover, examination of pigment translocation in this cell type clearly implicated the cytoskeleton [Walker et al, 1985;Palazzo et al, 1989a], and suggested that as observed in other chromatophores [Rozdzial and Haimo, 1986], phosphorylation might be an important regulator of organelle transport [Lynch et al, 1986; reviewed by Tchen et al, 1988;Palazzo et al, 1989b].…”

Actin‐dependent carotenoid droplet dispersion in permeabilized cultured goldfish xanthophores

“…In the past few years, cell permeabilization has been widely and successfully applied in numerous studies of cell motility including mitosis [Cande et al, 19811, secretion [Lee and Holz, 1986;Perrin et al, 19871, and pigment organelle translocations [Tchen et al, 1988;Rozdzial and Haimo, 1986a,b;Ogawa et al, 1986;Stearns and Ochs, 1982;McNiven and Ward, 1988;Clark and Rosenbaum, 19821. A permeabilized plasma membrane allows the introduction of agents normally impermeable to the plasma membrane and pharmacological and biochemical manipulations that are not feasible with intact cells.…”

Morphological studies on the mechanisms of pigmentary organelle transport in fish xanthophores and melanophores

“…Molecular players in dispersion and aggregation were eludicated in permeabilized cell and cell-free systems (Lynch et al, 1981(Lynch et al, , 1986aPalazzo et al, 1989b;Tchen et al, 1988;Wu et al, 1990;Yu et al, 1989Yu et al, , 1990Zeng et al, 1989) along with the immunologic characterization of intermediate filaments (Walker et al, 1989) and the CD phosphoprotein, p57 (Gu, 1991). In order to carefully study the cytoskeleton, xanthophores were extracted and fixed after inducing different states of pigment distribution.…”