pEg2 aurora-A kinase, histone H3 phosphorylation, and chromosome assembly in Xenopus egg extract

In eukaryotes cell division is accompanied by phosphorylation of histone H3 at serine 10. In this work we have studied the kinase activity responsible for this histone H3 modification by using cell-free extracts prepared from Xenopus eggs. We have found that the Xenopus aurora-A kinase pEg2, immunop recipitated from the extract, is able to phosphorylate specifically histone H3 at serine 10. The enzyme is incorporated into chromatin during in vitro chromosome assembly, and the, kinetics of this incorporation parallels tha t of histone H3 phosphorylation. Recombinant pEg2 phosphorylates efficientl y histone H3 at serine 10 in reconstituted nucleosomes and in sperm nuclei decondensed in heated extracts. These data identify pEg2 as a good candidat e for mitotic histone H3 kinase. However, immunodepletion of pEg2 does not interfere with the chromosome assembly properties of the extract nor with t he pattern of H3 phosphorylation, suggesting the existence of multiple kina ses involved in this H3 modification in Xenopus eggs. This hypothesis is su pported by in gel activity assay experiments using extracts from Saccharomy ces cerevisiae.