Regionally regulated conversion of protein expression in the skin during anuran metamorphosis

The back skin of anuran larvae undergoes drastic metamorphic changes which are quite different from the tail skin. To have an insight into the mechanism of the region-dependent changes, we made a thorough survey of metamorphic alterations in protein expression in both the body and tail skin of Xenopus laevis tadpoles utilizing high-resolution two-dimensional electrophoresis. Approximately 400 proteins were detected, 9 of which were identified as known proteins and were used as index proteins to standardize the readout of distribution of protein spots on gels. The overall pattern of polypeptide distribution (map) was classified into "larval type" and "adult type." The maps showed stage-specific changes. Up to stage 58 the back skin map remained the larval type, showed remarkable changes for the first time at stage 60 (whose pattern was a combination of both larval and adult types), and became the adult type at stage 62. The basic feature of the tail skin map remained the larval type through the climax stage of metamorphosis. Experiments using tadpoles treated with an antithyroid drug showed that most of back skin proteins detected on the map are thyroid hormone responsive in their expression. Expression of keratin II (adult-type keratin) was found to be uniquely regulated. As expected, silver staining detected it in the back but not in the tail. However, the protein-labeling experiment with [35S]methionine showed that the tail skin is actively synthesizing the adult keratin, suggesting that tail cells have a specific elimination mechanism of the protein.