Figure 9.

Mos-induced interphase arrest is not due to inhibiton of Cdk2/cyclin E. Cycling extracts containing sperm chromatin were supplemented with buffer, Mos, or Mos plus recombinant Cdk2/cyclin E proteins. Left, Cdk2 kinase activity in the treated extracts at 90 min. Cdk2 complexes were immunoprecipitated with a Cdk2-specific antibody. The extracts with Mos alone showed at most a small reduction in activity while the extracts with additional Cdk2/cyclin E display an approximately 2.5-fold increase in measureable activity. Right, maximal percent nuclear envelope breakdown (NEBD) was scored in the same experiment through phase-contrast microscopy. The addition of excess Cdk2/cyclin E activity did not reverse the MAP kinase-induced G₂-phase arrest as assessed by phase-contrast microscopy.