Figure 3.

 Block of cell division in Xenopus embryos by the tetrameric hMAD2. (A) Twenty-five nanoliters of 10 mg/ml recombinant hMAD2^t and hMAD2^m proteins were microinjected into one of the blastomeres in two-cell stage Xenopus embryos. The embryos were collected and analyzed 2 hr after injection. Cell cycle arrest by hMAD2^t is stable for at least 8 hr before cells degenerate. (B) To assay histone H1 kinase assays, both blastomeres in two-cell stage embryos were injected with hMAD2, and extracts from injected embryos were analyzed for histone H1 kinase activity. Unfertilized eggs and equivalent aliquots of interphase and mitotic extracts were also assayed as controls.