SF3B14 is involved in the centrosome regulation through splicing of TUBGCP6 pre-mRNA.

Splicing precursor messenger RNA (pre-mRNA) is a critical step to produce physiologically functional protein. Splicing failure not only gives rise to dysfunctional proteins but also generates abnormal protein function, which causes several diseases. Several pre-mRNA splicing factors are reported to regulate mitosis directly at mitotic structures and/or indirectly through controlling the pre-mRNA splicing for mitotic proteins. In this study, we described the mitotic functions of SF3B14, a component of the spliceosomal U2 small nuclear ribonucleoprotein (snRNP), which we identified as a candidate involved in mitosis based on the large-scale RNA interference (RNAi) screen of the nucleolar proteome database. We observed that SF3B14 depletion caused prolonged mitosis and several mitotic defects, such as monopolar spindle and chromosome misalignment during metaphase. Although SF3B14 was found in the nucleolar proteome database, our immunofluorescent stainings demonstrated that SF3B14 was predominantly localized in the nucleoplasm and excluded from the nucleolus during interphase. In addition, SF3B14 did not colocalize with specific mitotic structures during mitosis, which is not in line with its direct mitotic function. Notably, we found that the SF3B14 depletion reduced protein levels of TUBGCP6, required for centrosome regulation, and increased the unspliced/spliced ratio of its mRNA. Taken together, we propose that the pre-mRNA of TUBGCP6 is one of the targets for SF3B14 splicing through which SF3B14 controls mitotic chromosome behavior.

Identification of a novel nucleolar protein complex required for mitotic chromosome segregation through centromeric accumulation of Aurora B.

The nucleolus is a membrane-less nuclear structure that disassembles when cells undergo mitosis. During mitosis, nucleolar factors are thus released from the nucleolus and dynamically change their subcellular localization; however, their functions remain largely uncharacterised. Here, we found that a nucleolar factor called nucleolar protein 11 (NOL11) forms a protein complex with two tryptophan-aspartic acid (WD) repeat proteins named WD-repeat protein 43 (WDR43) and Cirhin in mitotic cells. This complex, referred to here as the NWC (NOL11-WDR43-Cirhin) complex, exists in nucleoli during interphase and translocates to the periphery of mitotic chromosomes, i.e., perichromosomal regions. During mitotic progression, both the congression of chromosomes to the metaphase plate and sister chromatid cohesion are impaired in the absence of the NWC complex, as it is required for the centromeric enrichment of Aurora B and the associating phosphorylation of histone H3 at threonine 3. These results reveal the characteristics of a novel protein complex consisting of nucleolar proteins, which is required for regulating kinetochores and centromeres to ensure faithful chromosome segregation.

Nucleolar integrity during interphase supports faithful Cdk1 activation and mitotic entry.

The nucleolus is a dynamic nuclear body that has been demonstrated to disassemble at the onset of mitosis; the relationship between cell cycle progression and nucleolar integrity, however, remains poorly understood. We studied the role of nucleolar proteins in mitosis by performing a global analysis using small interfering RNAs specific to nucleolar proteins; we focused on nucleolar protein 11 (NOL11), with currently unknown mitotic functions. Depletion of NOL11 delayed entry into the mitotic phase owing to increased inhibitory phosphorylation of cyclin-dependent kinase 1 (Cdk1) and aberrant accumulation of Wee1, a kinase that phosphorylates and inhibits Cdk1. In addition to effects on overall mitotic phenotypes, NOL11 depletion reduced ribosomal RNA (rRNA) levels and caused nucleolar disruption during interphase. Notably, mitotic phenotypes found in NOL11-depleted cells were recapitulated when nucleolar disruption was induced by depletion of rRNA transcription factors or treatment with actinomycin D. Furthermore, delayed entry into the mitotic phase, caused by the depletion of pre-rRNA transcription factors, was attributable to nucleolar disruption rather than to G2/M checkpoint activation or reduced protein synthesis. Our findings therefore suggest that maintenance of nucleolar integrity during interphase is essential for proper cell cycle progression to mitosis via the regulation of Wee1 and Cdk1.

The hierarchical structure of the perichromosomal layer comprises Ki67, ribosomal RNAs, and nucleolar proteins.

The perichromosomal layer (PCL) is a structure that surrounds mitotic chromosomes, found in both animal and plant cells. It comprises various proteins and RNAs, mainly derived from the nucleolus. Several functions for the PCL have been suggested; however, the mechanism of PCL organization during mitosis remains unclear. The localization of several nucleolar proteins to the PCL is reportedly dependent on pre-ribosomal RNAs and the marker of proliferation, Ki67, which is a major PCL-localized protein. Here we demonstrate that, although the removal of pre-ribosomal RNAs from the PCL causes PCL delocalization of several nucleolar proteins, it does not affect the localization of Ki67. Conversely, Ki67 depletion results in the dissociation of both pre-ribosomal RNAs and nucleolar proteins from the PCL, which indicates that Ki67 is required for the PCL accumulation of pre-ribosomal RNAs, to which several nucleolar proteins are associated. Given these findings, we propose a model for PCL organization that comprises three essential layers: the scaffolding protein Ki67, pre-ribosomal RNAs for linkage, and outer nucleolar proteins.