Activities of topoisomerase I in its complex with SRSF1

Human DNA topoisomerase I (topo I) catalyzes DNA relaxation and phosphorylates SRSF1. Whereas the structure of topo I complexed with DNA has been resolved, the structure of topo I in the complex with SRSF1 and structural determinants of topo I activities in this complex are not known. The main obstacle to resolving the structure is a contribution of unfolded domains of topo I and SRSF1 in formation of the complex. To overcome this difficulty, we employed a three-step strategy: identifying the interaction regions, modeling the complex, and validating the model with biochemical methods. The binding sites in both topo I and SRSF1 are localized in the structured regions as well as in the unfolded domains. One observes cooperation between the binding sites in topo I but not in SRSF1. Our results indicate two features of the unfolded RS domain of SRSF1 containing phosphorylated residues that are critical for the kinase activity of topo I: its spatial arrangement relative to topo I and the organization of its sequence. The efficiency of phosphorylation of SRSF1 depends on the length and flexibility of the spacer between the two RRM domains that uniquely determine an arrangement of the RS domain relative to topo I. The spacer also influences inhibition of DNA nicking, a prerequisite for DNA relaxation. To be phosphorylated, the RS domain has to include a short sequence recognized by topo I. A lack of this sequence in the mutants of SRSF1 or its spatial inaccessibility in SRSF9 makes them inadequate as topo I/kinase substrates.     

Sulforaphane, a cruciferous vegetable-derived isothiocyanate, inhibits protein synthesis in human prostate cancer cells

Sulforaphane (SFN) is a compound derived from cruciferous plants. Its anticancer properties have been demonstrated both, in cancer cell lines as well as tumors in animal models. It has been shown that SFN inhibits cell proliferation, induces apoptosis, autophagy, and sensitizes cancer cells to therapies. As induction of catabolic processes is often related to perturbation in protein synthesis we aimed to investigate the impact of SFN on this process in PC-3 human prostate cancer cells. In the present study we show that SFN inhibits protein synthesis in PC-3 cells in a dose- and time-dependent manner which is accompanied by a decreased phosphorylation of mTOR substrates. Translation inhibition is independent of mitochondria-derived ROS as it is observed in PC-3 derivatives devoid of functional mitochondrial respiratory chain (Rho0 cells). Although SFN affects mitochondria and slightly decreases glycolysis, the ATP level is maintained on the level characteristic for control cells. Inhibition of protein synthesis might be a protective response of prostate cancer cells to save energy. However, translation inhibition contributes to the death of PC-3 cells due to decreased level of a short-lived protein, survivin. Overexpression of this anti-apoptotic factor protects PC-3 cells against SFN cytotoxicity. Protein synthesis inhibition by SFN is not restricted to prostate cancer cells as we observed similar effect in SKBR-3 breast cancer cell line.     

Tyr‐Asp inhibition of glyceraldehyde 3‐phosphate dehydrogenase affects plant redox metabolism

How organisms integrate metabolism with the external environment is a central question in biology. Here, we describe a novel regulatory small molecule, a proteogenic dipeptide Tyr‐Asp, which improves plant tolerance to oxidative stress by directly interfering with glucose metabolism. Specifically, Tyr‐Asp inhibits the activity of a key glycolytic enzyme, glyceraldehyde 3‐phosphate dehydrogenase (GAPC), and redirects glucose toward pentose phosphate pathway (PPP) and NADPH production. In line with the metabolic data, Tyr‐Asp supplementation improved the growth performance of both Arabidopsis and tobacco seedlings subjected to oxidative stress conditions. Moreover, inhibition of Arabidopsis phosphoenolpyruvate carboxykinase (PEPCK) activity by a group of branched‐chain amino acid‐containing dipeptides, but not by Tyr‐Asp, points to a multisite regulation of glycolytic/gluconeogenic pathway by dipeptides. In summary, our results open the intriguing possibility that proteogenic dipeptides act as evolutionarily conserved small‐molecule regulators at the nexus of stress, protein degradation, and metabolism.     

Hyperthermic stress stimulates the association of both constitutive and inducible isoforms of 70 kDa heat shock protein with rat liver glucocorticoid receptor

Glucocorticoid hormone receptor exists in the cytoplasm of target cells in the form of dynamic multiprotein heterocomplexes with heat shock proteins Hsp90 and Hsp70, and additional components of the molecular chaperone machinery. Whole body hyperthermic stress was previously shown to induce alterations in protein composition of these complexes increasing the share of Hsp70, but participation of individual Hsp70 family members was not investigated. In the present study the association of glucocorticoid receptor with constitutive and inducible forms of Hsp70 in the liver cytosol of rats exposed to 41 degrees C whole body hyperthermic stress was examined. Immunoprecipitation of glucocorticoid receptor heterocomplexes by monoclonal anti-receptor antibody (BuGR2) followed by quantitative immunoblotting revealed the presence of both nucleocytoplasmic Hsp70 family members, constitutive--Hsc70 and inducible--Hsp72, within the complexes. Immediately after the stress only Hsc70 was found in association with glucocorticoid receptor. However, after the induction of Hsp72 by stress, its appearance within the glucocorticoid receptor heterocomplexes was also recorded and the presence of both Hsp70 forms within the heterocomplexes was evident by the end of examined 24h period after the stress. This study confirms that heat stress affects protein composition of rat liver glucocorticoid receptor heterocomplexes increasing the share of Hsp70 and shows that this increase could be equally ascribed to constitutive and inducible forms of Hsp70.     

A role for the common GTP-binding protein in coupling of chromosome replication to cell growth and cell division

Homologues of CgtA, the common GTP-binding protein of Vibrio harveyi, are present in diverse organisms ranging from bacteria to humans. In bacteria, proteins homologous to CgtA form a subfamily of small GTP-binding proteins, called Obg/Gtp1. Similarity between bacterial members of this subfamily and their eukaryotic homologues is as high as about 50%. Nevertheless, specific functions of these proteins remain largely unknown. Genes coding for CgtA-like proteins are essential in almost all species of bacteria. The only known exception is V. harveyi, whose cells survive disruption of the cgtA gene. Therefore, the V. harveyi cgtA insertional mutant is a very useful tool for studies on functions of CgtA. Here we demonstrate that under normal growth conditions, cells of the cgtA mutant are slightly larger than wild-type cells, whereas indirect inhibition of DNA replication initiation by addition of rifampicin results in significantly higher differences in average cell size between these two strains as measured by flow cytometry. These differences decreased when cell division was inhibited by cephalexin. DNA synthesis per cell mass was found to be increased in the cgtA mutant relative to wild-type V. harveyi strain, whereas the mutant cells grew slower than bacteria with functional cgtA gene. Kinetics of DNA replication after inhibition of cell division was also considerably different in wild-type and cgtA mutant strains. These results suggest that the cgtA gene product plays a role in coupling of DNA replication to cell growth and cell division.     

Expression of cyclin A,B1 and D1 after induction of cell cycle arrest in the Jurkat cell line exposed to doxorubicin

Jurkat human lymphoblastoid cells were incubated in increasing concentrations of doxorubicin (0.05, 0.1 and 0.15 μM) to induce cell death, and their expression of cyclin A, B1 and D1 was evaluated by flow cytometry (cell cycle progression, Annexin V assay, percentages and levels of each of the cyclins), transmission electron microscopy (ultrastructure) and confocal fluorescence microscopy (expression and intracellular localization of cyclins). After low‐dose doxorubicin treatment, Jurkat cells responded mainly by G2/M arrest, which was related to increased cyclin B1, A and D1 levels, a low level of apoptosis and/or mitotic catastrophe. The influence of doxorubicin on levels and/or localization of selected cyclins was confirmed, which may in turn contribute to the G2/M arrest induced by the drug.