The p90 ribosomal S6 kinase 2 specifically affects mitotic progression by regulating the basal level, distribution and stability of mitotic spindles

RSK2, also known as RPS6KA3 (ribosomal protein S6 kinase, 90 kDa, polypeptide 3), is a downstream kinase of the mitogen-activated protein kinase (MAPK) pathway, which is important in regulating survival, transcription, growth and proliferation. However, its biological role in mitotic progression is not well understood. In this study, we examined the potential involvement of RSK2 in the regulation of mitotic progression. Interestingly, depletion of RSK2, but not RSK1, caused the accumulation of mitotic cells. Time-lapse analysis revealed that mitotic duration, particularly the duration for metaphase-to-anaphase transition was prolonged in RSK2-depleted cells, suggesting activation of spindle assembly checkpoint (SAC). Indeed, more BubR1 (Bub1-related kinase) was present on metaphase plate kinetochores in RSK2-depleted cells, and depletion of BubR1 abolished the mitotic accumulation caused by RSK2 depletion, confirming BubR1-dependent SAC activation. Along with the shortening of inter-kinetochore distance, these data suggested that weakening of the tension across sister kinetochores by RSK2 depletion led to the activation of SAC. To test this, we analyzed the RSK2 effects on the stability of kinetochore–microtubule interactions, and found that RSK2-depleted cells formed less kinetochore–microtubule fibers. Moreover, RSK2 depletion resulted in the decrease of basal level of microtubule as well as an irregular distribution of mitotic spindles, which might lead to observed several mitotic progression defects such as increase in unaligned chromosomes, defects in chromosome congression and a decrease in pole-to-pole distance in these cells. Taken together, our data reveal that RSK2 affects mitotic progression by regulating the distribution, basal level and the stability of mitotic spindles.

Research progress on spindle assembly checkpoint gene BubR1 / 浙江大学学报·医学版

BubR1 gene is a homologue of the mitotic checkpoint gene Mad3 in budding yeast which is highly conserved in mammalian. BubR1 protein is a key component mediating spindle assembly checkpoint activation. BubR1 safeguards accurate chromosome segregation during cell division by monitoring kinetochore-microtubule attachments and kinetochore tension. There is a dose-dependent effect between the level of BubR1 expression and the function of spindle assembly checkpoint. BubR1-deficient would lead to mitotic progression with compromised spindle assembly checkpoint because cells become progressively aneuploid. Recently, it has been reported that BubR1 also plays important roles in meiotic, DNA damage response, cancer, infertility, and early aging. This review briefly summarizes the current progresses in studies of BubR1 function.

Sec13 induces genomic instability in U2OS cells

Sec13p has been known as an endoplasmic reticulum-Golgi transport protein. Recently, it has also been shown to be required for the formation of septation in the fission yeast Schizosaccharomyces pombe. In the present study, we focused on the role of a human homolog of Saccharomyces cerevisiae SEC13, Sec13 protein during mitosis in U2OS cells. We found that the expression of Sec13 was constant throughout the cell cycle, and localized to the kinetochores at metaphase during mitosis. By using green fluorescent protein technology, we observed that Sec13 is required for evasion of mitotic arrest in response to spindle damage, leading to G1-like phase and apoptotic cell death. In addition, cells expressing exogenous Sec13 showed giant nuclei compared to endogenous ones in the absence of nocodazole. These results demonstrate that Sec13 is involved in the regulation of the metaphase/anaphase transition and may be functionally associated with mitotic machinery to maintain genomic stability during mitosis.

The centromere: kinetochore complex.

At the meta-anaphase transition the centromeres in a genome separate in non-random sequential manner. This sequential separation depends upon the timing of replication of DNA located in the pericentric and centromeric region. Cells in long term cultures as well as some newborn humans carry dicentric chromosomes. The inactive centromeres in these dicentric chromosomes do not show any sequence of separation. Whether or not a dicentric chromosome would segregate equationally depends upon if only one centromere binds to microtubules or both are functional. In man and other higher apes, a 171 base pair long DNA repeat (the alphoid sequence) is present on all centromeres. In mouse, the minor satellite fraction is said to constitute the centromere. These two DNAs also carry a 17 bp long sequence, the CENP-B 'box' to which the CENP-B antigen is bound. Other species-eg, rat, pig, fish, Chinese hamster-exhibit still different sequences at the centromere and do not carry the CENP-B 'box' even though the antigen is ubiquitously present in all species. It is not clear why so many diverse sequences constitute the centromere when all centromeres look alike and perform the same function. I propose that the primary constriction owes its property not necessarily to its DNA composition but to some stereophysical property, eg the curvature and that the region is held together till late metaphase-anaphase due to a specific proteinaceous factor. The mammalian centromeres bind a complex of several proteins dubbed as CENtromere Proteins (CENP's). This complex, however, is not what constitutes the trilamellar kinetochore structure as see under the electron microscope.(ABSTRACT TRUNCATED AT 250 WORDS)