Characterization of a chromatin remodelling activity in Xenopus oocytes

The yeast SWI2/SNF2 protein is a component of a large protein complex which is involved in the remodelling of chromatin during transcriptional activation. Several homologous complexes have been found in Drosophila and mammals. We have examined the expression of the SWI2/SNF2 homologue BRG1 in Xenopus laevis using two antisera originally raised against the C-terminus of the rat and the human BRG1 protein. These two antisera cross-reacted with a protein found in both Xenopus liver and Xenopus oocytes. The Xenopus BRG1-like protein is expressed throughout oogenesis (stages I-VI) and embryogenesis. By injecting an expression vector containing the full-length human BRG1 cDNA into Xenopus oocytes, the relative molecular weight (Mr) of the Xenopus BRG1-like protein was shown to be slightly lower than that of the human BRG1, 190 000 and 200 000, respectively. The Xenopus BRG1-like protein elutes at a Mr of approximately 2 000 000 on Superose HR6trade mark size-exclusion chromatography, indicating that it is part of a larger complex, as are all other known SWI/SNF proteins. Nucleosome remodelling activity was co-eluted with the BRG1 immunogenic activity in both ion-exchange and size-exclusion chromatography.