Biochemical characterization of a CDC6-like protein from the crenarchaeon Sulfolobus solfataricus

Cdc6 proteins play an essential role in the initiation of chromosomal DNA replication in Eukarya. Genes coding for putative homologs of Cdc6 have been also identified in the genomic sequence of Archaea, but the properties of the corresponding proteins have been poorly investigated so far. Herein, we report the biochemical characterization of one of the three putative Cdc6-like factors from the hyperthermophilic crenarchaeon Sulfolobus solfataricus (SsoCdc6-1). SsoCdc6-1 was overproduced in Escherichia coli as a His-tagged protein and purified to homogeneity. Gel filtration and glycerol gradient ultracentrifugation experiments indicated that this protein behaves as a monomer in solution (molecular mass of about 45 kDa). We demonstrated that SsoCdc6-1 binds single- and double-stranded DNA molecules by electrophoretic mobility shift assays. SsoCdc6-1 undergoes autophosphorylation in vitro and possesses a weak ATPase activity, whereas the protein with a mutation in the Walker A motif (Lys-59 --> Ala) is completely unable to hydrolyze ATP and does not autophosphorylate. We found that SsoCdc6-1 strongly inhibits the ATPase and DNA helicase activity of the S. solfataricus MCM protein. These findings provide the first in vitro biochemical evidence of a functional interaction between a MCM complex and a Cdc6 factor and have important implications for the understanding of the Cdc6 biological function.     

Reaction and substrate specificity of recombinant pig kidney Dopa decarboxylase under aerobic and anaerobic conditions

Dopa decarboxylase (DDC) catalyzes as main reaction the stereospecific CO(2) abstraction from L-Dopa and L-5-hydroxytryptophan (5-HTP), generating the corresponding aromatic amines, dopamine and serotonin, respectively. Side reactions with turnover time of minutes are also catalyzed by the enzyme. In particular, DDC exhibits half-transaminase activity toward D-aromatic amino acids and oxidative deaminase activity toward aromatic amines. The latter reaction could represent a new activity for this class of enzymes. Studies on the effect exerted by O(2) on reaction specificity of DDC revealed that under anaerobic conditions decarboxylation of L-aromatic amino acids takes place with a k(cat) approximately half of that measured in the presence of O(2), and is accompanied by a decarboxylation-dependent transamination, whereas oxidative deamination of aromatic amines is replaced by half-transamination. Half-transamination of D-aromatic amino acids is unaffected by the presence or absence of O(2). Some structural elements relevant for the control of reaction and substrate specificity of DDC have been identified by means of limited tryptic digestion and site-directed mutagenesis studies. All together, the data indicate that the chemical nature of the substrate, the presence of O(2), the integrity of a mobile loop, the absence of perturbation in the coenzyme-binding cleft and pH are important requirements for the achievement of a closed conformational state where the highest level of reaction specificity is reached.     

Treponema denticola cystalysin catalyzes beta-desulfination of L-cysteine sulfinic acid and beta-decarboxylation of L-aspartate and oxalacetate

Pyridoxal 5'-phosphate-dependent cystalysin from Treponema denticola catalyzes the beta-displacement of the beta-substituent from both L-aspartate and L-cysteine sulfinic acid. The steady-state kinetic parameters for beta-desulfination of L-cysteine sulfinic acid, k(cat) and K(m), are 89+/-7 s(-1) and 49+/-9 mM, respectively, whereas those for beta-decarboxylation of L-aspartate are 0.8+/-0.1 s(-1) and 280+/-70 mM. Moreover, cystalysin in the pyridoxamine 5'-phosphate form has also been found to catalyze beta-decarboxylation of oxalacetate as shown by consumption of oxalacetate and a concomitant production of pyruvate. The k(cat) and K(m) of this reaction are 0.15+/-0.01 s(-1) and 13+/-2 mM, respectively. Possible mechanistic and physiological implications are discussed.     

Spectroscopic and kinetic analyses reveal the pyridoxal 5'-phosphate binding mode and the catalytic features of Treponema denticola cystalysin

To obtain insight into the functional properties of Treponema denticola cystalysin, we have analyzed the pH- and ligand-induced spectral transitions, the pH dependence of the kinetic parameters, and the substrate specificity of the purified enzyme. The absorption spectrum of cystalysin has maxima at 418 and 320 nm. The 320 nm band increases at high pH, while the 418 nm band decreases; the apparent pK(spec) of this spectral transition is about 8.4. Cystalysin emitted fluorescence at 367 and 504 nm upon excitation at 320 and 418 nm, respectively. The pH profile for the 367 nm emission intensity increases above a single pK of approximately 8.4. On this basis, the 418 and 320 nm absorbances have been attributed to the ketoenamine and substituted aldamine, respectively. The pH dependence of both log k(cat) and log k(cat)/K(m) for alpha,beta-elimination reaction indicates that a single ionizing group with a pK value of approximately 6.6 must be unprotonated to achieve maximum velocity. This implies that cystalysin is more catalytically competent in alkaline solution where a remarkable portion of its coenzyme exists as inactive aldamine structure. Binding of substrates or substrate analogues to the enzyme over the pH range 6-9.5 converts both the 418 and 320 nm bands into an absorbing band at 429 nm, assigned to the external aldimine in the ketoenamine form. All these data suggest that the equilibrium from the inactive aldamine form of the coenzyme shifts to the active ketoenamine form on substrate binding. In addition, reinvestigation of the substrate spectrum of alpha,beta-elimination indicates that cystalysin is a cyst(e)ine C-S lyase rather than a cysteine desulfhydrase as claimed previously.     

Modular organization of the Sulfolobus solfataricus mini-chromosome maintenance protein

Mini-chromosome maintenance (MCM) proteins form ring-like hexameric complexes that are commonly believed to act as the replicative DNA helicase at the eukaryotic/archaeal DNA replication fork. Because of their simplified composition with respect to the eukaryotic counterparts, the archaeal MCM complexes represent a good model system to use in analyzing the structural/functional relationships of these important replication factors. In this study the domain organization of the MCM-like protein from Sulfolobus solfataricus (Sso MCM) has been dissected by trypsin partial proteolysis. Three truncated derivatives of Sso MCM corresponding to protease-resistant domains were produced as soluble recombinant proteins and purified: the N-terminal domain (N-ter, residues 1-268); a fragment comprising the AAA+ and C-terminal domains (AAA+-C-ter, residues 269-686); and the C-terminal domain (C-ter, residues 504-686). All of the purified recombinant proteins behaved as monomers in solution as determined by analytical gel filtration chromatography, suggesting that the polypeptide chain integrity is required for stable oligomerization of Sso MCM. However, the AAA+-C-ter derivative, which includes the AAA+ motor domain and retains ATPase activity, was able to form dimers in solution when ATP was present, as analyzed by size exclusion chromatography and glycerol gradient sedimentation analyses. Interestingly, the AAA+-C-ter protein could displace oligonucleotides annealed to M13 single-stranded DNA although with a reduced efficiency in comparison with the full-sized Sso MCM. The implications of these findings for understanding the DNA helicase mechanism of the MCM complex are discussed.     

The human GINS complex binds to and specifically stimulates human DNA polymerase alpha-primase

The eukaryotic GINS complex has an essential role in the initiation and elongation phases of genome duplication. It is composed of four paralogous subunits--Sld5, Psf1, Psf2 and Psf3--which are ubiquitous and evolutionarily conserved in eukaryotic organisms. Here, we report the biochemical characterization of the human GINS complex (hGINS). The four hGINS subunits were coexpressed in Escherichia coli in a highly soluble form and purified as a complex. hGINS was shown to interact directly with the heterodimeric human DNA primase, by using either surface plasmon resonance measurements or by immunoprecipitation experiments carried out with anti-hGINS antibodies. The DNA polymerase alpha-primase synthetic activity was specifically stimulated by hGINS on various primed DNA templates. The significance of these findings is discussed in view of the molecular dynamics at the human replication fork.     

Reciprocally interacting domains of protein phosphatase 1 and focal adhesion kinase

The Murine double-minute clone 2 (Mdm2) onco-protein is the principal regulator of the tumour suppressor, p53. Mdm2 acts as an E3-type ubiquitin ligase that mediates the ubiquitylation and turnover of p53 under normal, unstressed circumstances. In response to cellular stress, such as DNA damage, the Mdm2–p53 interaction is disrupted. Part of the mechanism of uncoupling p53 from Mdm2-mediated degradation involves hypo-phosphorylation of a cluster of phosphorylated serine residues in the central acidic domain of Mdm2. Here, we show that two of the residues within this domain that are phosphorylated in vivo, Ser-260 and Ser-269, are phosphorylated by CK2 in vitro. Treatment of cells with the CK2 inhibitor, 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), leads to the induction of p53 and downstream targets of p53 including Mdm2 itself and p21. These data are consistent with the idea that CK2-mediated phosphorylation of Mdm2 may regulate Mdm2-mediated p53 turnover.     

Heat shock proteins in animal neoplasms and human tumours—a comparison

Heat shock proteins (HSPs) are implicated in all phases of cancer from proliferation, impaired apoptosis and sustained angiogenesis to invasion and metastasis. The presence of abnormal HSP levels in several human tumours suggests that these proteins could be used as diagnostic and/or prognostic markers, whilst the direct correlation between HSP expression and drug resistance in neoplastic tissues means they could also be used to predict cancer response to specific treatment. HSPs have also been successfully targeted in clinical trials modifying their expression or chaperone activity. Preliminary studies in veterinary medicine have also demonstrated the presence of altered HSP expression in neoplasms, and the study of carcinogenesis and the role of HSPs in animal models will surely be an additional source of information for clinical cancer research.