p10 single-stranded nucleic acid binding protein from murine leukemia virus binds metal ions via the peptide sequence Cys sup 26 -X sub 2 -Cys sup 29 -X sub 4 -His sup 34 -X sub 4 -Cys sup 39

The RNA binding protein of 56 residues encoded by the extreme 3{prime} region of the gag gene of Rauscher murine leukemia virus (MuLV) has been chemically synthesized by a solid-phase synthesis approach. Since the peptide contains a Cys{sup 26}-X{sub 2}-Cys{sup 29}-X{sub 4}-His{sup 34}-X{sub 2}-Cys{sup 39} sequence that is shared by all retroviral gag polyproteins which as been proposed to be a metal binding region, it was of considerable interest to examine the metal binding properties of the complete p10 protein. As postulated, p10 binds the metal ions Cd(II), Co(II), and Zn(II). The Co(II) protein shows a set of d-d absorption bands typical of a tetrahedral Co(II) complex. The ultraviolet absorption spectrum also indicates Cd(II) binding. The {sup 113}Cd NMR spectrum of {sup 113}Cd(II)-p10 reveals one signal at {delta} = 648 ppm. This chemical shift correlates well with that predicted for ligation of {sup 113}Cd(II) to three -S{sup {minus}} from the three Cys residues of p10. The chemical shift of {sup 113}Cd(II)-p10 changes by only 4 ppm upon binding of d(pA){sub 6}, indicating that the chelate complex is little changed by oligonucleotide binding. Circular dichroism studies reveal that the apo-p10 protein has a secondary structure that contain only {approximately} 20% {beta}-sheet. Zn(II) binding does not appear to induce significant additional secondary structure. Fluorescence titrations carried out with poly(r{epsilon}A) show that the affinity of p10 for this RNA is not significantly altered by the presence of Cd(II), Co(II), or Zn(II). Although p10 clearly binds metal ions in vitro, metal ion coordination does not appear to result in any major change in p10 structure or in its nonspeciific RNA binding properties.