Retinoblastoma (RB-1) is a tumor suppressor gene that encodes a 105-kDa nuclear phosphoprotein. To date, RB genes have been isolated only from metazoans. We have isolated a cDNA from maize endosperm whose predicted protein product (ZmRb) shows homology to the "pocket" A and B domains of the Rb protein family. We found ZmRb behaves as a pocket protein based on its ability to specifically interact with oncoproteins encoded by DNA tumor viruses (E7, T-Ag, E1A). ZmRb can interact in vitro and in vivo with the replication-associated protein, RepA, encoded by the wheat dwarf virus. The maize Rb-related protein undergoes changes in level and phosphorylation state concomitant with endoreduplication, and it is phosphorylated in vitro by an S-phase kinase from endoreduplicating endosperm cells. Together, our results suggest that ZmRb is a representative of the pocket protein family and may play a role in cell cycle progression. Moreover, certain plant monopartite geminiviruses may operate similarly to mammalian DNA viruses, by targeting and inactivating the retinoblastoma protein, which otherwise induces G1 arrest.Retinoblastoma (RB-1) is a tumor suppressor gene that encodes a nuclear phosphoprotein with a molecular mass of about 105 kDa (1, 2). Inactivation of the RB-1 gene contributes to both familial and sporadic forms of cancer (3). Retinoblastoma and its related p107 and p130 proteins are among the negative regulators of the cell cycle (4, 5). While hypophosphorylated, the Rb protein exerts a growth suppressive effect and arrests cells in G1 phase.Hyperphosphorylation of Rb, or its interaction with viral oncoproteins, prevents Rb from performing its normal functions at G1 phase and enables cells to begin DNA synthesis (6).
Endoreduplication is an endonuclear chromosome duplication that occurs in the absence of mitosis and in Zea mays (L.) is required for endosperm development. Induction of DNA synthesis during early stages of endosperm development is maintained by increasing the amount and activity of S phase-related protein kinases, which was demonstrated here by their ability to interact with human E2F or with the adenovirus E1A proteins. In addition it was shown that endoreduplicated endosperm cells contain an inhibitor that suppresses the activity of the M phase-promoting factor (MPF). These results demonstrate that in maize endosperm, endoreduplication proceeds as a result of two events, inhibition of MPF and induction of S phase-related protein kinases.
SummaryCytosine methylation at symmetrical CpG and CpNpG sequences plays a key role in the epigenetic control of plant growth and development; yet, the way by which the methylation signal is interpreted into a functional state has not been elucidated. In animals, the methylation signal is recognized by methylCpG-binding domain (MBD) proteins that speci®cally bind methylated CpG dinucleotides. In Arabidopsis thaliana, 12 putative MBD proteins were identi®ed and classi®ed into seven subclasses. Here, we characterized six MBD proteins representing four subclasses (II, III, IV, and VI) of the Arabidopsis MBD family. We found that AtMBD7 (subclass VI), a unique protein containing a double MBD motif, as well as AtMBD5 and AtMBD6 (subclass IV), bind speci®cally symmetrically methylated CpG sites. The MBD motif derived from AtMBD6, but not from AtMBD2, was suf®cient for binding methylated CpG dinucleotides. AtMBD6 precipitated histone deacetylase (HDAC) activity from the leaf nuclear extract. The examined AtMBD proteins neither bound methylated CpNpG sequences nor did they display DNA demethylase activity. Our results suggest that AtMBD5, AtMBD6, and AtMBD7 are likely to function in Arabidopsis plants as mediators of the CpG methylation, linking DNA methylation-induced gene silencing with histone deacetylation.
The remarkable regenerative capacity displayed by plants and various vertebrates, such as amphibians, is largely based on the capability of somatic cells to undergo dedifferentiation. In this process, mature cells reverse their state of differentiation and acquire pluripotentiality--a process preceding not only reentry into the cell cycle but also a commitment for cell death or trans- or redifferentiation. Recent studies provide a new perspective on cellular dedifferentiation, establishing chromatin reorganization as its fundamental theme.
Cells in maize (Zea mays) endosperm undergo multiple cycles of endoreduplication, with some attaining DNA contents as high as 96C and 192C. Genome amplification begins around 10 d after pollination, coincident with cell enlargement and the onset of starch and storage protein accumulation. Although the role of endoreduplication is unclear, it is thought to provide a mechanism that increases cell size and enhances gene expression. To investigate this process, we reduced endoreduplication in transgenic maize endosperm by ectopically expressing a gene encoding a dominant negative mutant form of cyclin-dependent kinase A. This gene was regulated by the 27-kD γ-zein promoter, which restricted synthesis of the defective enzyme to the endoreduplication rather than the mitotic phase of endosperm development. Overexpression of a wild-type cyclin-dependent kinase A increased enzyme activity but had no effect on endoreduplication. By contrast, ectopic expression of the defective enzyme lowered kinase activity and reduced by half the mean C-value and total DNA content of endosperm nuclei. The lower level of endoreduplication did not affect cell size and only slightly reduced starch and storage protein accumulation. There was little difference in the level of endosperm gene expression with high and low levels of endoreduplication, suggesting that this process may not enhance transcription of genes associated with starch and storage protein synthesis
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