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.