Mitosis is a complicated cellular process involving extensive structural reorganizations in many subcellular compartments and a sequence of highly orchestrated events. The temporal and spatial changes in mitotic cells are tightly regulated to ensure high fidelity of genomic transmission during cell division. Mitosis is initiated by accumulation of active kinase complexes formed between mitotic cyclins (cyclin A and B in human) and master mitosis regulator CDC2 (or CDK1) at the G2/M transition [1
]. The expression of many other mitosis regulators also peaks during G2/M phase, some of which share the common control by transcription factors such as Fox M1 and the DREAM complex [2
]. Chromosome condensation appears during prophase, concurring with reorganization of microtubule cytoskeleton into mitotic spindles and separation of duplicated centrosomes to opposite sides of the nucleus. The activity of cyclin A/CDC2 lasts until nuclear envelope breakdown when cyclin A is degraded [4
]. The cyclin B/CDC2 complex, together with many other chromosome and microtubule associated proteins, promotes the formation of bipolar spindle and chromosome congression to the metaphase plate [1
]. Cyclin B is then destructed after ubiquitylation by the anaphase promoting complex/cyclosome (APC/C) [1
]. Loss of cyclin B/CDC2 activity ensures unidirectional progression of mitosis [6
]. Sister chromatids separate and move to the opposite spindle poles after anaphase onset. Chromosomes then decondense, and the nuclear envelope reforms during telophase. When cytokinesis is completed, the abscission occurs at the midbody between two daughter cells, the spindle is disassembled and cells flatten out into interphase morphology again.
The centromeres are specialized loci on chromosomes that form primary constrictions during mitosis. There are currently 18 known human proteins constitutively associating with centromeres throughout the cell cycle [7
]. The kinetochores are macromolecular protein complexes built upon centromeres to connect with spindle microtubules [8
]. Kinetochores are dynamic structures that are assembled and disassembled at each sister chromatid during each and every mitosis [8
]. Kinetochores also harbour activities contributing to chromosome movement throughout mitosis and the spindle assembly checkpoint (SAC) [5
]. The SAC monitors kinetochore-microtubule attachment status to inhibit activation of the APC/C until the metaphase-to-anaphase transition. Under electron microscope centromeres and kinetochores are contiguous structurally, and both play important roles in regulating chromosome segregation. We will use the term “the centromere/kinetochore complex” in this paper to reflect the intertwined relationship between the two subcellular structures.
Although the knowledge of proteins localized at the centromere/kinetochore complex has increased exponentially in the past few years [7
], we still have much to learn about the proteins that contribute to the spatial and temporal regulation of mitosis. As indicated in recent reports, without a “parts list” of critical molecular players, it is impossible to reach a comprehensive understanding about mitosis and its connections with tumorigenesis and cancer drug effects [9
]. Recent genomics and proteomics research has revealed that proteins in the same functional modules are usually transcriptionally co-expressed and/or organized into clusters in PPI networks (e.g. [13
]). Many publicly available bioinformatics resources have deposited data obtained from large scale co-expression profiling and PPI studies, and provided tools to retrieve and organize the data.
As an end user interested in taking advantage of the huge amount of genomics and proteomics data in the public databases, we first conducted exhaustive literature review and compiled a list of 196 human centromere/kinetochore proteins, and selected 64 among them as “core” components with well-characterized mitotic functions. By interrogating and integrating available online resources using the 64 core proteins as a query group, we have then identified potential novel mitosis regulators among top-ranking genes/proteins that co-express or interact with the 64 core centromere/kinetochore components. Experimental validation has identified one novel kinetochore protein TRIP13 and two novel centrosomal proteins KIAA1377 and DDX39.