Radiolysis of lac Repressor by γ-Rays and Heavy Ions: A Two-Hit Model for Protein Inactivation

• Published in: Biophysical journal 2002-05, Vol.82 (5), p.2373-2382
• Main Authors: Charlier, Michel, Eon, Séverine, Sèche, Édouard, Bouffard, Serge, Culard, Françoise, Spotheim-Maurizot, Mélanie
• Format: Article
• Language: English
• Subjects: Argon; Bacterial Proteins - antagonists & inhibitors; Bacterial Proteins - chemistry; Bacterial Proteins - radiation effects; Dimerization; Escherichia coli - physiology; Escherichia coli - radiation effects; Escherichia coli Proteins; Gamma Rays; Lac Repressors; Lactose - antagonists & inhibitors; Macromolecular Substances; Models, Biological; Models, Molecular; Peptides - chemistry; Protein Subunits; Repressor Proteins - antagonists & inhibitors; Repressor Proteins - chemistry; Repressor Proteins - radiation effects; Space life sciences
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/S0006-3495(02)75582-3

Upon γ-ray or argon ion irradiation of the lac repressor protein, its peptide chain is cleaved and the protein loses its lac operator-binding activity, as shown respectively by polyacrylamide gel electrophoresis and retardation gel electrophoresis. We developed phenomenological models that satisfactorily account for the experimental results: the peptide chain cleavage model considers that the average number of chain breaks per protomer is proportional to the irradiation dose and that the distribution of the number of breaks per protomer obeys Poisson’s law. The repressor inactivation model takes into account the quaternary structure (a dimer of dimer) and the organization of the repressor in domains (two DNA binding sites, one per dimer). A protomer is inactivated by at least two different radiation-induced damages. A dimer is inactivated when at least one of the two protomers is inactivated. A tetramer is inactivated when both dimers are inactivated. From the combination of both models, we can deduce that chain cleavage cannot account for the protein inactivation, which should mainly result from oxidation of amino acid side chains. Indeed, particularly oxidizable and accessible amino acids (Tyr, His) are involved in the DNA binding process.