alpha-Melanotropin-induced changes in protein phosphorylation in melanophores

To investigate a possible role of protein phosphorylation in the mechanism of action of alpha-MSH, excised tail-fins of Xenopus tadpoles were incubated with or without alpha-MSH. After homogenization, in vitro endogenous protein phosphorylation was assayed using [gamma-32P]ATP. alpha-MSH treatment of intact tail-fins, producing full pigment dispersion, resulted in a 5-fold increase in 32P-incorporation into a 53 kDa protein band. This increase in 53 kDa phosphorylation was completely reversible. The increase was not found in homogenates from the melanophore-free part of the alpha-MSH-treated tail-fins. Phosphorylation of the 53 kDa protein could be detected in homogenates of alpha-MSH-treated primary cultured melanophores. Incubation of tail-fins with ACTH1-24, an alpha-MSH-like peptide producing full pigment dispersion, also induced an increase in 53 kDa phosphorylation. A structurally related peptide (ACTH15-24) and an unrelated peptide (LH-RH), neither of which induced pigment dispersion, were ineffective in stimulating 53 kDa phosphorylation. Injection of white adapted tadpoles with 1 micrograms of alpha-MSH or adaptation of tadpoles to a black background also resulted in a significant increase in 53 kDa phosphorylation. alpha-MSH added to the homogenates did not affect 53 kDa phosphorylation, indicating that alpha-MSH acts through a receptor-mediated mechanism. The increase in 53 kDa phosphorylation measured in vitro (post hoc), most likely reflects an alpha-MSH-induced decrease in 53 kDa phosphorylation in vivo. Our results strongly suggest that a decrease in 53 kDa phosphorylation is involved in the mechanism of action of alpha-MSH on melanophores.