SEDIMENTATION ANALYSES OF THE SALT-INDUCED AND DIVALENT METAL ION-INDUCED OLIGOMERIZATION OF NUCLEOLAR PROTEIN B23

Protein B23 is a major nonribosomal nucleolar protein and putative rib osome assembly factor that has been demonstrated to form oligomers. Se dimentation velocity and equilibrium analyses were used to examine the oligomerization properties of recombinant proteins B23.1 and B23.2. U nder low ionic strength conditions protein B23.1 was predominantly a 2 .1S monomer with small amounts of a 7.1S oligomer. At NaCl concentrati ons of 0.1 M and above the protein was almost exclusively the 7.1 S ol igomer. The oligomer remained the predominant species in NaCl concentr ations as high as 1 M, suggesting that oligomers are not stabilized by electrostatic interactions. Low concentrations of divalent mel:al ion s (0.1 -1 mM Ca2+ or Mg2+) also promoted oligomerization. Reducing age nts had no effect on oligomerization, indicating that disulfide bridge s are not important in oligomer formation. Protein B23.2, the carboxyl -terminal truncated isoform, had sedimentation characteristics similar to that of protein B23.1, suggesting that the carboxyl-terminal end o f protein B23.1 is not essential for oligomerization. Protein B23.1 wa s previously shown to bind nucleic acids [Wang, D., Baumann, A., Szebe ni, A., & Olson, M. O. J. (1995) J. Biol. Chem. 269, 30994-30998]. The effect of protein B23.1 oligomerization on its interaction with a 230 base pair DNA fragment was examined by sedimentation analyses. Under conditions where significant amounts of monomer were present? protein B23.1 was capable of binding DNA, whereas conditions that strongly fav ored oligomerization caused a nearly complete abolition of DNA binding activity. These studies suggest that protein B23 exists in an equilib rium between monomer and oligomer and that the quaternary structure of the protein may regulate its DNA binding properties.