ATPase and DNA helicase activities of the Saccharomyces cerevisiae anti-recombinase Srs2

Saccharomyces cerevisiae SRS2 encodes an ATP-dependent DNA helicase that is needed for DNA damage checkpoint responses and that modulates the efficiency of homologous recombination. Interestingly, strains simultaneously mutated for SRS2 and a variety of DNA repair genes show low viability that can be overcome by inactivating homologous recombination, thus implicating inappropriate recombination as the cause of growth impairment in these mutants. Here, we report on our biochemical characterization of the ATPase and DNA helicase activities of Srs2. ATP hydrolysis by Srs2 occurs efficiently only in the presence of DNA, with ssDNA being considerably more effective than dsDNA in this regard. Using homopolymeric substrates, the minimal DNA length for activating ATP hydrolysis is found to be 5 nucleotides, but a length of 10 nucleotides is needed for maximal activation. In its helicase action, Srs2 prefers substrates with a 3' ss overhang, and approximately 10 bases of 3' overhanging DNA is needed for efficient targeting of Srs2 to the substrate. Even though a 3' overhang serves to target Srs2, under optimized conditions blunt-end DNA substrates are also dissociated by this protein. The ability of Srs2 to unwind helicase substrates with a long duplex region is enhanced by the inclusion of the single-strand DNA-binding factor replication protein A.     

Glycosylphosphatidylinositol-anchored ceruloplasmin is required for iron efflux from cells in the central nervous system

Ceruloplasmin (Cp) is a ferroxidase that converts highly toxic ferrous iron to its non-toxic ferric form. A glycosylphosphatidylinositol (GPI)-anchored form of this enzyme is expressed by astrocytes in the mammalian central nervous system, whereas the secreted form is expressed by the liver and found in serum. Lack of this enzyme results in iron accumulation in the brain and neurodegeneration. Herein, we show using astrocytes purified from the central nervous system of Cp-null mice that GPI-Cp is essential for iron efflux and not involved in regulating iron influx. We also show that GPI-Cp colocalizes on the astrocyte cell surface with the divalent metal transporter IREG1 and is physically associated with IREG1. In addition, IREG1 alone is unable to efflux iron from astrocytes in the absence of GPI-Cp or secreted Cp. We also provide evidence that the divalent metal influx transporter DMT1 is expressed by astrocytes and is likely to mediate iron influx into these glial cells. The coordinated actions of GPI-Cp and IREG1 may be required for iron efflux from neural cells, and disruption of this balance could lead to iron accumulation in the central nervous system and neurodegeneration.     

Novel inhibitors of advanced glycation endproducts

A number of natural or synthetic compounds as AGE inhibitors have been proposed, discovered or currently being advanced by others and us. We have identified two new classes of aromatic compounds; aryl- (and heterocyclic) ureido and aryl (and heterocyclic) carboxamido phenoxyisobutyric acids, and benzoic acid derivatives and related compounds, as potential inhibitors of glycation and AGE formation. Some of these novel compounds also showed "AGE-breaking" activities in vitro. Current evidence is that chelation of transition metals and/or trapping or indirect inhibition of formation of reactive carbonyl compounds are involved in the mechanisms of action of these novel AGE inhibitors and breakers. Here, we review the inhibitors of glycation and AGE-breakers published to date and present the results of our in vitro and in vivo investigations on a number of these novel AGE inhibitors. These AGE-inhibitors and AGE-breakers may find therapeutic use in the treatment of diseases that AGE formation and accumulation may be responsible for their pathogenesis such as diabetes, Alzheimer's, rheumatoid arthritis, and atherosclerosis.     

Prediction of tyrosine sulfation sites in animal viruses

Post-translational modification of proteins by tyrosine sulfation enhances the affinity of extracellular ligand-receptor interactions important in the immune response and other biological processes in animals. For example, sulfated tyrosines in polyomavirus and varicella-zoster virus may help modulate host cell recognition and facilitate viral attachment and entry. Using a Position-Specific-Scoring-Matrix with an accuracy of 96.43%, we analyzed the possibility of tyrosine sulfation in all 1517 animal viruses available in the Swiss-Prot database. From a total of 97,729 tyrosines, we predicted 5091 sulfated tyrosine sites from 1024 viruses. Our site predictions in hemagglutinin of influenza A, VP4 of rotavirus, and US28 of cytomegalovirus strongly suggest an important link between tyrosine sulfation and viral disease mechanisms. In each of these three viral proteins, we observed highly conserved amino acid sequences surrounding predicted sulfated tyrosine sites. Tyrosine sulfation appears to be much more common in animal viruses than is currently recognized.     

A human TERT C-terminal polypeptide sensitizes HeLa cells to H2O2-induced senescence without affecting telomerase enzymatic activity

We have constructed a 27-kDa hTERT C-terminal polypeptide (hTERTC27) devoid of domains required for telomerase activity and demonstrated that it is capable of nuclear translocation/telomere-end targeting. Here we showed that expression of a low level of hTERTC27 renders hTERT positive HeLa cells sensitive to H(2)O(2)-induced oxidative stress and subsequent cell senescence. The senescence-associated gene, the cyclin/cdk inhibitor p21(Waf1), was up-regulated. This occurs without changing the expression of endogenous hTERT, causing significant telomere shortening or inhibiting telomerase activity. Results from this study suggest for the first time that in addition to telomerase activity, the C-terminus of hTERT also plays a role in hTERT-mediated cellular resistance to oxidative stress.     

Human leukocyte formin: a novel protein expressed in lymphoid malignancies and associated with Akt

The very large family of Formin proteins is involved in processes such as morphogenesis, embryonic differentiation, cell polarity, and cytokinesis. A novel human gene from the Formin family, denominated human leukocyte formin gene, was cloned. The cDNA of the gene was determined to be 3959bp long with an open reading frame of 3302bp and computational analysis located this gene on chromosome 17, suggesting that it is composed of 27 exons. Northern blot analysis revealed a restricted expression of mRNA in the thymus, spleen, and peripheral blood leukocytes in normal human tissues. Western blot analysis demonstrated that the protein encoded by this gene is overexpressed in lymphoid malignancies; cancer cell lines and peripheral blood leukocyte from chronic lymphocytic leukemia (CLL) patients. Furthermore, the human leukocyte formin protein was observed to associate with Akt, a critical survival regulator in many different cell types.     

The J-domain of Hsp40 couples ATP hydrolysis to substrate capture in Hsp70

The Escherichia coli Hsp40 DnaJ uses its J-domain to target substrate polypeptides for binding to the Hsp70 DnaK, but the mechanism of J-domain function has been obscured by a substrate-like interaction between DnaJ and DnaK. ATP hydrolysis in DnaK is associated with a conformational change that captures the substrate, and both DnaJ and substrate can stimulate ATP hydrolysis. However, substrates cannot trigger capture by DnaK in the presence of ATP, and substrates stimulate a DnaK conformational change that is uncoupled from ATP hydrolysis. The role of the J-domain was examined using the fluorescent derivative of a fusion protein composed of the J-domain and a DnaK-binding peptide. In the absence of ATP, DnaK-binding affinity of the fusion protein is similar to that of the unfused peptide. However, in the presence of ATP, the affinity of the fusion protein is dramatically increased, which is opposite to the decrease in DnaK affinity typically exhibited by peptides. Binding of a fusion protein that contains a defective J-domain is insensitive to ATP. According to results from isothermal titration calorimetry, the J-domain binds to the DnaK ATPase domain with weak affinity (K(D) = 23 microM at 20 degrees C). The interaction is characterized by a positive enthalpy, small heat capacity change (DeltaC(p)= -33 kcal mol(-1)), and increasing binding affinity for increasing temperatures in the physiological range. In conditions that support binding of the J-domain to the ATPase domain, the J-domain accelerates ATP hydrolysis and a simultaneous conformational change in DnaK that is associated with peptide capture. The defective J-domain is inactive, despite the fact that it binds to the DnaK ATPase domain with higher than wild-type affinity. The results are most consistent with an allosteric mechanism of J-domain action in which the J-domain couples ATP hydrolysis to peptide capture by accelerating ATP hydrolysis and delaying DnaK closure until ATP is hydrolyzed.