The retinoblastoma susceptibility gene was the first tumor suppressor gene identified in humans and the first tumor suppressor gene knocked out by targeted deletion in mice. RB serves as a transducer between the cell cycle machinery and promoter-specific transcription factors, its most documented activity being the repression of the E2F family of transcription factors, which regulate the expression of genes involved in cell proliferation and survival. Recent investigations of RB function suggest that it works as a fundamental regulator to coordinate pathways of cellular growth and differentiation. In this review, we unravel the novel role of an equally important aspect of RB in downregulating the differentiation inhibitor EID-1 during cellular differentiation by teasing apart the signal, which elicit differentiation and limit cell cycle progression, since the molecular mechanisms relating to RB activation of differentiation is much less understood. We review the various roles for RB in differentiation of neurons, muscle, adipose tissue, and the retina. In addition, we provide an update for the current models of the role of RB in cell cycle to entry and exit, extending the view toward chromatin remodeling and expose the dichotomies in the regulation of RB family members. We conclude with a discussion of a novel RB regulatory network, incorporating the dynamic contribution of EID family proteins.

Chromokinesins are microtubule-motor molecules that possess chromatin binding activity and are important for mitotic and meiotic regulation. The chromokinesin-member Kif4A is unique in that it localizes to nucleus during interphase of the cell cycle. Kif4 deletion by gene targeting in mouse embryonic cells was known to associate with DNA damage response. However, its precise role in DNA damage or repair pathway is not clear. Here we report that Kif4A associates with BRCA2 in a biochemical identification and that the interaction is mediated by the Kif4A C-terminal cargo-binding domain and BRCA2 C-terminal conserved region. Upon nucleus-specific laser micro-irradiation, Kif4A was rapidly recruited to sites of DNA damage. Significantly, the depletion of Kif4A from cells by shRNA impaired the ionizing-radiation induced foci (IRIF) formation of Rad51, both quantitatively and qualitatively. In contrast, the IRIF of γ-H2AX or NBS1 was largely intact. Moreover, Kif4A knockdown rendered cells hypersensitive to ionizing radiation in a colonogenic survival assay. We further demonstrated that Kif4A deficiency led to significantly decreased homologous recombination in an I-SceI endonuclease induced in vivo recombination assay. Together, our results suggest a novel role for a chromokinesin family member in the DNA damage response by modulating the BRCA2/Rad51 pathway.

In yeast mitochondria, RNA degradation takes place through the coordinated activities of ySuv3 helicase and yDss1 exoribonuclease (mtEXO), whereas in bacteria, RNA is degraded via RNaseE, RhlB, PNPase, and enolase. Yeast lacking the Suv3 component of the mtEXO form petits and undergo a toxic accumulation of omega intron RNAs. Mammalian mitochondria resemble their prokaryotic origins by harboring a polyadenylation-dependent RNA degradation mechanism, but whether SUV3 participates in regulating RNA turnover in mammalian mitochondria is unclear. We found that lack of hSUV3 in mammalian cells subsequently yielded an accumulation of shortened polyadenylated mtRNA species and impaired mitochondrial protein synthesis. This suggests that SUV3 may serve in part as a component of an RNA degradosome, resembling its yeast ancestor. Reduction in the expression levels of oxidative phosphorylation components correlated with an increase in reactive oxygen species generation, whereas membrane potential and ATP production were decreased. These cumulative defects led to pleiotropic effects in mitochondria such as decreased mtDNA copy number and a shift in mitochondrial morphology from tubular to granular, which eventually manifests in cellular senescence or cell death. Thus, our results suggest that SUV3 is essential for maintaining proper mitochondrial function, likely through a conserved role in mitochondrial RNA regulation.Prokaryotes and eukaryotes both utilize RNA processing and degradation, albeit via different pathways, as one mechanism for controlling gene expression (1-4). The RNA degradosome of Escherichia coli, yeast, and mammalian cells have all conserved the ability to turnover RNA; however, the assembly of their multicomponent machineries differ (5, 6). In E. coli, RNA degradation is conducted by the cooperation of four principal enzymes (7). The first enzyme is an endoribonuclease, RNase E, which initiates the turnover of many RNAs by cleaving singlestranded RNA internally and is essential for cell growth (2,8,9). The second enzyme is a 50-kDa DEAD-box helicase, RhlB, which unwinds and translocates RNA substrates (10). RhlB facilitates the degradation of structured mRNA decay intermediates by the third 85-kDa enzyme, polynucleotide polymerase (PNPase).3 This process, requiring ATP hydrolysis, is believed to involve the local unwinding of structures that block PNPase, thereby ensuring rapid degradation by PNPase (11-13). The fourth enzyme, a 48-kDa glycolytic enzyme enolase, associates with RNase E in response to stress caused by the overabundance of phosphosugars (14, 15). However, enolase is a unique and mysterious contributor to the degradosome, as it is not an RNA-binding protein nor does it appear to be directly involved in RNA turnover.In Saccharomyces cerevisiae, mitochondrial RNA degradation also requires the activity of multifunctional components (mtEXO) (16), which consist mainly of two enzymes: ySuv3 and yDss1 (17)(18)(19)(20). Yeast Suv3 is a nuclear gene-encoded protein that localizes to mitochondria in ...

The basement membrane, thought to be a barrier to cell movement, contains flexible perforations that can serve as cellular gateways.