ABSTRACT. We previously reported that exogenous histone H1, when injected into mitotic cells, disrupts the synchronous progression of mitotic events by delaying chromosome decondensation. This strategy was utilized to determine whether any other interphase proteins are also able to disrupt normal mitotic processes, when introduced into the mitotic phase. We found that a chromatin subfraction from bovine liver nuclei induced postmitotic micronuclei formation in a dose-dependent manner when injected into the prometaphase of rat kangaroo kidney epithelial (PtK2) cells. Close observation showed that, in the case of injected mitotic cells, the mitotic spindles were disrupted, chromosomes became scattered throughout the cytoplasm, and actin filaments were organized ectopically. In addition, when the fraction was injected into interphase cells, extra actin filaments were formed and microtubule organization was affected. In order to determine whether the micronuclei formation resulted from the ectopic formation of actin filaments, we examined the effect of the actin polymerization inhibitor, cytochalasin D. The results showed that the drug inhibited micronuclei formation. From these findings, we concluded that this chromatin subfraction contains actin polymerization activity, thus causing the disruption of mitotic spindles.Key words: cell cycle/chromosome segregation/actin filament polymerization/mitotic spindle The cell nucleus of higher eukaryotes undergoes dramatic structural and functional changes during the cell cycle, with such significant cell functions as replication of DNA, transcription and maturation of mRNA occurring in interphase. In this phase, chromosomes decondense and are surrounded by a nuclear envelope that is perforated by nuclear pores, through which nucleo-cytoplasmic transport occurs.In the mitotic phase, the higher order structures of chromosomes are changed into compact ones that are suitable for correct segregation. The interphase microtubules (MTs) disassemble to form a mitotic spindle (MS), which is a bipolar structure composed of microtubules and their associated proteins. The sister chromatids, the centromeres of which bind to the MS extending from bipolar microtubule organizing centers (MTOCs), segregate from one another. The nuclear envelope, including nuclear pore complexes, disassembles and some of its components are dispersed throughout the cytoplasm. At this phase, no DNA and little RNA are synthesized. At the end of mitosis, chromosomes decondense, the nuclear envelope reassembles to form two daughter nuclei, and the MS dissociates and is reassembled into interphase MTs.We previously demonstrated that chromosome condensation is prolonged, sister chromatid separation is inhibited and the synchronous progression of mitotic events is disrupted when the intracellular level of histone H1 is elevated at mitosis by the injection of exogeneous H1 (Matsuoka et al., 1994). In this study, we report that a chromatin subfraction derived from bovine liver nuclei is capable of inducing micronuclei ...