Carcinogenesis, Vol. 21, No. 5, 1051-1057,
May 2000
© 2000 Oxford University Press
Carcinogenesis |
Cadmium mutagenicity and human nucleotide excision repair protein XPA: CD, EXAFS and 1H/15N-NMR spectroscopic studies on the zinc(II)- and cadmium(II)-associated minimal DNA-binding domain (M98–F219)
Pacific Northwest National Laboratories, Environmental Molecular Sciences Laboratory and Biogeochemistry Resources, Richland, WA 99352, USA
Human XPA is a 31 kDa protein involved in nucleotide excision repair (NER), a ubiquitous, multi-enzyme pathway responsible for processing multiple types of DNA damage in the eukaryotic genome. A zinc-associated, C4-type motif (C105-X2-C108-X17-C126-X2-C129) located in the minimal DNA-binding region (M98–F219) of XPA (XPA-MBD) is essential for damaged DNA recognition. Cadmium is a known carcinogen and can displace the zinc in many metal-binding proteins. It has been suggested that the carcinogenic properties of cadmium may result from structural changes effected in XPA when Cd2+ is substituted for Zn2+ in the metal-binding site. The solution structure of XPA-MBD containing zinc(II) has recently been determined [Buchko et al., (1998) Nucleic Acids Res., 26, 2779–2788; Buchko et al., (1999) Biochemistry, 38, 15116–15128]. To assess the effects of cadmium(II) substitution on the structure of XPA-MBD, XPA-MBD was expressed in minimal medium supplemented with cadmium acetate to yield a protein that was almost exclusively (>95%) associated with cadmium(II) (CdXPA-MBD). Extended X-ray absorption fine structure spectra collected on ZnXPA-MBD and CdXPA-MBD in frozen (77 K) 15% aqueous glycerol solution show that the metal is coordinated to the sulfur atoms of four cysteine residues with an average metal–sulfur bond length of 2.34 ± 0.01 and 2.54 ± 0.01 Å, respectively. Comparison of the circular dichroism, two-dimensional 1H,15N-HSQC, and three-dimensional 15N-edited HSQC–NOESY spectra of ZnXPA-MBD and CdXPA-MBD show that there are no structural differences between the two proteins. The absence of major structural changes upon substituting cadmium(II) for zinc(II) in XPA suggests that cadmium-induced mutagenesis is probably not due to structural perturbations to the zinc-binding core of XPA.
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