Radiolysis of lac repressor by gamma-rays and heavy ions: a two-hit model for protein inactivation

Radiation damage to nucleoprotein complexes in macromolecular crystallography

Significant progress has been made in macromolecular crystallography over recent years in both the understanding and mitigation of X-ray induced radiation damage when collecting diffraction data from crystalline proteins. In contrast, despite the large field that is productively engaged in the study of radiation chemistry of nucleic acids, particularly of DNA, there are currently very few X-ray crystallographic studies on radiation damage mechanisms in nucleic acids. Quantitative comparison of damage to protein and DNA crystals separately is challenging, but many of the issues are circumvented by studying pre-formed biological nucleoprotein complexes where direct comparison of each component can be made under the same controlled conditions. Here a model protein-DNA complex C.Esp1396I is employed to investigate specific damage mechanisms for protein and DNA in a biologically relevant complex over a large dose range (2.07-44.63 MGy). In order to allow a quantitative analysis of radiation damage sites from a complex series of macromolecular diffraction data, a computational method has been developed that is generally applicable to the field. Typical specific damage was observed for both the protein on particular amino acids and for the DNA on, for example, the cleavage of base-sugar N1-C and sugar-phosphate C-O bonds. Strikingly the DNA component was determined to be far more resistant to specific damage than the protein for the investigated dose range. At low doses the protein was observed to be susceptible to radiation damage while the DNA was far more resistant, damage only being observed at significantly higher doses.

Polycyanoacrylate porous material for bone tissue substitution

A proof of concept study has been conducted for the design of a porous biodegradable material containing nanocapsules and two actives with independent release-bimodal drug-eluting implants. Completely safe synthetic material free from risk of prion and virus contamination was tested in vivo, and a method for controlling the rate of biodegradation of poly-2-cyanoacrylic polymer was developed. Novel perfluorinated 2-cyanoacrylic esters have been applied for the chemical modification of polyethyl-2-cyanoacrlylate copolymers. Internal imide-cycle formation has been used to retard the rate of enzymatic hydrolysis of the 2-cyanoacrylic copolymer main chain.

Radiation-induced oxidative damage to the DNA-binding domain of the lactose repressor

Understanding the cellular effects of radiation-induced oxidation requires the unravelling of key molecular events, particularly damage to proteins with important cellular functions. The Escherichia coli lactose operon is a classical model of gene regulation systems. Its functional mechanism involves the specific binding of a protein, the repressor, to a specific DNA sequence, the operator. We have shown previously that upon irradiation with gamma-rays in solution, the repressor loses its ability to bind the operator. Water radiolysis generates hydroxyl radicals (OH* radicals) which attack the protein. Damage of the repressor DNA-binding domain, called the headpiece, is most likely to be responsible of this loss of function. Using CD, fluorescence spectroscopy and a combination of proteolytic cleavage with MS, we have examined the state of the irradiated headpiece. CD measurements revealed a dose-dependent conformational change involving metastable intermediate states. Fluorescence measurements showed a gradual degradation of tyrosine residues. MS was used to count the number of oxidations in different regions of the headpiece and to narrow down the parts of the sequence bearing oxidized residues. By calculating the relative probabilities of reaction of each amino acid with OH. radicals, we can predict the most probable oxidation targets. By comparing the experimental results with the predictions we conclude that Tyr7, Tyr12, Tyr17, Met42 and Tyr47 are the most likely hotspots of oxidation. The loss of repressor function is thus correlated with chemical modifications and conformational changes of the headpiece.

High-LET irradiation of a DNA-binding protein: protein-protein and DNA-protein crosslinks

The chromosomal protein MC1 is a monomeric protein of 93 amino acids that is able to bind any DNA but has a slight preferential affinity for some sequences and structures, like cruciform and minicircles. The protein has been irradiated with 36Ar18+ ions of 95 MeV/nucleon. The LET of these particles in water is close to 270 keV/microm. We tested the activity of the protein by measuring its ability to form complexes with DNA. We tested the integrity of the protein by measuring the molecular weight of the species formed. Compared with gamma radiation, we observed for the same dose a less efficient inactivation of the protein, a greater protection of the protein by the bound DNA, a lower induction of chain breakage, and a greater production of protein-protein and DNA-protein crosslinks. The results are discussed in terms of the quantitative and the qualitative differences between the two types of radiation: The global radical yield is slightly higher with gamma rays, whereas the density of radicals produced along the particle track is considerably higher with argon ions.

Radiation Abolishes Inducer Binding to Lactose Repressor

The lactose operon functions under the control of the repressor-operator system. Binding of the repressor to the operator prevents the expression of the structural genes. This interaction can be destroyed by the binding of an inducer to the repressor. If ionizing radiations damage the partners, a dramatic dysfunction of the regulation system may be expected. We showed previously that gamma irradiation hinders repressor-operator binding through protein damage. Here we show that irradiation of the repressor abolishes the binding of the gratuitous inducer isopropyl-1-beta-D-thiogalactoside (IPTG) to the repressor. The observed lack of release of the repressor from the complex results from the loss of the ability of the inducer to bind to the repressor due to the destruction of the IPTG binding site. Fluorescence measurements show that both tryptophan residues located in or near the IPTG binding site are damaged. Since tryptophan damage is strongly correlated with the loss of IPTG binding ability, we conclude that it plays a critical role in the effect. A model was built that takes into account the kinetic analysis of damage production and the observed protection of its binding site by IPTG. This model satisfactorily accounts for the experimental results and allows us to understand the radiation-induced effects.

Response of a DNA-binding protein to radiation-induced oxidative stress

The DNA-binding protein MC1 is a chromosomal protein extracted from the archaebacterium Methanosarcina sp. CHTI55. It binds any DNA, and exhibits an enhanced affinity for some short sequences and structures (circles, cruciform DNA). Moreover, the protein bends DNA strongly at the binding site. MC1 was submitted to oxidative stress through gamma-ray irradiation. In our experimental conditions, damage is essentially due to hydroxyl radicals issued from water radiolysis. Upon irradiation, the regular complex between MC1 and DNA disappears, while a new complex appears. In the new complex, the protein loses its ability to recognise preferential sequences and DNA circles, and bends DNA less strongly than in the regular one. The new complex disappears and the protein becomes totally inactivated by high doses.A model has been proposed to explain these experimental results. Two targets, R(1) and R(2), are concomitantly destroyed in the protein, with different kinetics. R(2) oxidation has no effect on the regular binding, whereas R(1) oxidation modifies the functioning of MC1: loss of preferential site and structure recognition, weaker bending. The destruction of both R(1) and R(2) targets leads to a total inactivation of the protein. This model accounts for the data obtained by titrations of DNA with irradiated proteins. When the protein is irradiated in the complex with DNA, bound DNA protects its binding site on the protein very efficiently. The highly oxidisable tryptophan and methionine could be the amino acid residues implicated in the inactivation process.