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 laevis, contain mobile melanin filled organelles, and are well suited for studies on organelle movement. The intracellular regulation of the movement seems to be controlled by serine and threonine phosphorylations and dephosphorylations. Melatonin induces aggregation of the melanosomes to the cell centre through a G(i/o)-protein-coupled receptor, Melic, which leads to an inhibition of PKA and a stimulation of PP2A. However, this study shows that the melatonin-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 phosphorylations, showed inhibited melanosome movement after melatonin stimulation, and a lower degree of tyrosine phosphorylation of the similar to 280 kDa protein. The adenylyl cyclase activator forskolin, and the G(i/o) protein inhibitor pertussis toxin, also inhibited tyrosine phosphorylation of the similar to 280 kDa protein. The results indicate that melatonin stimulation generates tyrosine phosphorylation of a high molecular weight protein, an event that seems to be essential for melanosome aggregation. (C) 2000 Elsevier Science Inc. All rights reserved.