Melatonin-induced organelle movement in melanophores is coupled to tyrosine phosphorylation of a high molecular weight protein

Melanophores, brown to black pigment cells from, for example, Xenopus laevi s, contain mobile melanin filled organelles, and are well suited for studie s on organelle movement. The intracellular regulation of the movement seems to be controlled by serine and threonine phosphorylations and dephosphoryl ations. Melatonin induces aggregation of the melanosomes to the cell centre through a G(i/o)-protein-coupled receptor, Melic, which leads to an inhibi tion of PKA and a stimulation of PP2A. However, this study shows that the m elatonin-induced aggregation of melanosomes is also accompanied by tyrosine phosphorylation of a protein with a molecular weight of similar to 280 kDa . Cells pre-incubated with genistein, an inhibitor of tyrosine phosphorylat ions, showed inhibited melanosome movement after melatonin stimulation, and a lower degree of tyrosine phosphorylation of the similar to 280 kDa prote in. The adenylyl cyclase activator forskolin, and the G(i/o) protein inhibi tor pertussis toxin, also inhibited tyrosine phosphorylation of the similar to 280 kDa protein. The results indicate that melatonin stimulation genera tes tyrosine phosphorylation of a high molecular weight protein, an event t hat seems to be essential for melanosome aggregation. (C) 2000 Elsevier Sci ence Inc. All rights reserved.