Centromere regulation: new players, new rules, new questions

Histone storage and deposition in the early Drosophila embryo

Drosophila development initiates with the formation of a diploid zygote followed by the rapid division of embryonic nuclei. This syncytial phase of development occurs almost entirely under maternal control and ends when the blastoderm embryo cellularizes and activates its zygotic genome. The biosynthesis and storage of histones in quantity sufficient for chromatin assembly of several thousands of genome copies represent a unique challenge for the developing embryo. In this article, we have reviewed our current understanding of the mechanisms involved in the production, storage, and deposition of histones in the fertilized egg and during the exponential amplification of cleavage nuclei.

Repetitive centromeric satellite RNA is essential for kinetochore formation and cell division

SAT III RNA binds to the kinetochore component CENP-C and is required for correct assembly and function of the kinetochore at centromeres.

Chromosome segregation requires centromeres on every sister chromatid to correctly form and attach the microtubule spindle during cell division. Even though centromeres are essential for genome stability, the underlying centromeric DNA is highly variable in sequence and evolves quickly. Epigenetic mechanisms are therefore thought to regulate centromeres. Here, we show that the 359-bp repeat satellite III (SAT III), which spans megabases on the X chromosome of Drosophila melanogaster, produces a long noncoding RNA that localizes to centromeric regions of all major chromosomes. Depletion of SAT III RNA causes mitotic defects, not only of the sex chromosome but also in trans of all autosomes. We furthermore find that SAT III RNA binds to the kinetochore component CENP-C, and is required for correct localization of the centromere-defining proteins CENP-A and CENP-C, as well as outer kinetochore proteins. In conclusion, our data reveal that SAT III RNA is an integral part of centromere identity, adding RNA to the complex epigenetic mark at centromeres in flies.

The E3 ligase CUL3/RDX controls centromere maintenance by ubiquitylating and stabilizing CENP-A in a CAL1-dependent manner

Centromeres are defined by the presence of the histone H3 variant CENP-A in a subset of centromeric nucleosomes. CENP-A deposition to centromeres depends on a specialized loading factor from yeast to humans that is called CAL1 in Drosophila. Here, we show that CAL1 directly interacts with RDX, an adaptor for CUL3-mediated ubiquitylation. However, CAL1 is not a substrate of the CUL3/RDX ligase but functions as an additional substrate-specifying factor for the CUL3/RDX-mediated ubiquitylation of CENP-A. Remarkably, ubiquitylation of CENP-A by CUL3/RDX does not trigger its degradation but stabilizes CENP-A and CAL1. Loss of RDX leads to a rapid degradation of CAL1 and CENP-A and to massive chromosome segregation defects during development. Essentially, we identified a proteolysis-independent role of ubiquitin conjugation in centromere regulation that is essential for the maintenance of the centromere-defining protein CENP-A and its loading factor CAL1.

Mediator promotes CENP-a incorporation at fission yeast centromeres

At Schizosaccharomyces pombe centromeres, heterochromatin formation is required for de novo incorporation of the histone H3 variant CENP-A(Cnp1), which in turn directs kinetochore assembly and ultimately chromosome segregation during mitosis. Noncoding RNAs (ncRNAs) transcribed by RNA polymerase II (Pol II) directs heterochromatin formation through not only the RNA interference (RNAi) machinery but also RNAi-independent RNA processing factors. Control of centromeric ncRNA transcription is therefore a key factor for proper centromere function. We here demonstrate that Mediator directs ncRNA transcription and regulates centromeric heterochromatin formation in fission yeast. Mediator colocalizes with Pol II at centromeres, and loss of the Mediator subunit Med20 causes a dramatic increase in pericentromeric transcription and desilencing of the core centromere. As a consequence, heterochromatin formation is impaired via both the RNAi-dependent and -independent pathways, resulting in loss of CENP-A(Cnp1) from the core centromere, a defect in kinetochore function, and a severe chromosome segregation defect. Interestingly, the increased centromeric transcription observed in med20Δ cells appears to directly block CENP-A(Cnp1) incorporation since inhibition of Pol II transcription can suppress the observed phenotypes. Our data thus identify Mediator as a crucial regulator of ncRNA transcription at fission yeast centromeres and add another crucial layer of regulation to centromere function.