Arabidopsis S6 kinase mutants display chromosome instability and altered RBR1-E2F pathway activity.

The 40S ribosomal protein S6 kinase (S6K) is a conserved component of signalling pathways controlling growth in eukaryotes. To study S6K function in plants, we isolated single- and double-knockout mutations and RNA-interference (RNAi)-silencing lines in the linked Arabidopsis S6K1 and S6K2 genes. Hemizygous s6k1s6k2/++ mutant and S6K1 RNAi lines show high phenotypic instability with variation in size, increased trichome branching, produce non-viable pollen and high levels of aborted seeds. Analysis of their DNA content by flow cytometry, as well as chromosome counting using DAPI staining and fluorescence in situ hybridization, revealed an increase in ploidy and aneuploidy. In agreement with this data, we found that S6K1 associates with the Retinoblastoma-related 1 (RBR1)-E2FB complex and this is partly mediated by its N-terminal LVxCxE motif. Moreover, the S6K1-RBR1 association regulates RBR1 nuclear localization, as well as E2F-dependent expression of cell cycle genes. Arabidopsis cells grown under nutrient-limiting conditions require S6K for repression of cell proliferation. The data suggest a new function for plant S6K as a repressor of cell proliferation and required for maintenance of chromosome stability and ploidy levels.

Inhibition of cysteine protease activity disturbs DNA replication and prevents mitosis in the early mitotic cell cycles of sea urchin embryos.

Recent findings suggested that the role of cysteine proteases would not be limited to protein degradation in lysosomes but would also play regulatory functions in more specific cell mechanisms. We analyzed here the role of these enzymes in the control of cell cycle during embryogenesis. The addition of the potent cysteine protease inhibitor E64d to newly fertilized sea urchin eggs disrupted cell cycle progression, affecting nuclear as well as cytoplasmic characteristic events. Monitoring BrdU incorporation in E64d treated eggs demonstrated that DNA replication is severely disturbed. Moreover, this drug treatment inhibited male histones degradation, a step that is necessary for sperm chromatin remodeling and precedes the initiation of DNA replication in control eggs. This inhibition likely explains the DNA replication disturbance and suggests that S phase initiation requires cysteine protease activity. In turn, activation of the DNA replication checkpoint could be responsible for the consecutive block of nuclear envelope breakdown (NEB). However, in sea urchin early embryos this checkpoint doesn't control the mitotic cytoplasmic events that are not tightly coupled with NEB. Thus the fact that microtubule spindle is not assembled and cyclin B-cdk1 not activated under E64d treatment more likely rely on a distinct mechanism. Immunofluorescence experiments indicated that centrosome organization was deficient in absence of cysteine protease activity. This potentially accounts for mitotic spindle disruption and for cyclin B mis-localization in E64d treated eggs. We conclude that cysteine proteases are essential to trigger S phase and to promote M phase entry in newly fertilized sea urchin eggs.

During male pronuclei formation chromatin remodeling is uncoupled from nucleus decondensation.

Male pronucleus formation involves sperm nucleus decondensation and sperm chromatin remodeling. In sea urchins, male pronucleus decondensation was shown to be modulated by protein kinase C and a cdc2-like kinase sensitive to olomoucine in vitro assays. It was further demonstrated that olomoucine blocks SpH2B and SpH1 phosphorylation. These phosphorylations were postulated to participate in the initial steps of male chromatin remodeling during male pronucleus formation. At final steps of male chromatin remodeling, all sperm histones (SpH) disappear from male chromatin and are subsequently degraded by a cysteine protease. As a result of this remodeling, the SpH are replaced by maternal histone variants (CS). To define if sperm nucleus decondensation is coupled with sperm chromatin remodeling, we have followed the loss of SpH in zygotes treated with olomoucine. SpH degradation was followed with anti-SpH antibodies that had no cross-reactivity with CS histone variants. We found that olomoucine blocks SpH1 and SpH2B phosphorylation and inhibits male pronucleus decondensation in vivo. Interestingly, the normal schedule of SpH degradation remains unaltered in the presence of olomoucine. Taken together these results, it was concluded that male nucleus decondensation is uncoupled from the degradation of SpH associated to male chromatin remodeling. From these results, it also emerges that the phosphorylation of SpH2B and SpH1 is not required for the degradation of the SpH that is concurrent to male chromatin remodeling.