Functional interaction between the CSE2 gene product and centromeres in Saccharomyces cerevisiae

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.

Nutlin-3 affects expression and function of retinoblastoma protein: role of retinoblastoma protein in cellular response to nutlin-3

The retinoblastoma protein (Rb) plays a pivotal role in regulating cell proliferation and apoptosis. Nutlin-3, a small molecule MDM2 antagonist blocking interaction between MDM2 and p53, activates p53 resulting in cell growth arrest or apoptosis in various cancer cells. However, the molecular basis for the different cellular responses upon nutlin-3 treatment is not fully understood. In this study, we show that nutlin-3 activates p53 resulting in a dramatic increase in MDM2 expression and a marked reduction in total Rb protein levels. Interestingly, nutlin-3 reduces the levels of hypophosphorylated Rb and induces massive apoptosis in SJSA-1 cells, which can be largely rescued by knockdown of MDM2 or by expression of constitutively active Rb. By contrast, nutlin-3 treatment of several human cancer cells, including A549, U2-OS, and HCT116, results in an accumulation of hypophosphorylated Rb and cell cycle arrest but not apoptosis. Furthermore, we show that down-regulation of Rb by nutlin-3 does not lead to E2F1 activation nor does E2F1 play a critical role for nutlin-3-induced apoptosis in SJSA-1 cells. Taken together, these results suggest that Rb plays a critical role in influencing cellular response to activation of p53 pathway by nutlin-3.

A protein interaction map of the mitotic spindle

The mitotic spindle consists of a complex network of proteins that segregates chromosomes in eukaryotes. To strengthen our understanding of the molecular composition, organization, and regulation of the mitotic spindle, we performed a system-wide two-hybrid screen on 94 proteins implicated in spindle function in Saccharomyces cerevisiae. We report 604 predominantly novel interactions that were detected in multiple screens, involving 303 distinct prey proteins. We uncovered a pattern of extensive interactions between spindle proteins reflecting the intricate organization of the spindle. Furthermore, we observed novel connections between kinetochore complexes and chromatin-modifying proteins and used phosphorylation site mutants of NDC80/TID3 to gain insights into possible phospho-regulation mechanisms. We also present analyses of She1p, a novel spindle protein that interacts with the Dam1 kinetochore/spindle complex. The wealth of protein interactions presented here highlights the extent to which mitotic spindle protein functions and regulation are integrated with each other and with other cellular activities.