Identification and characterization of a hexameric form of nucleolar phosphoprotein B23

Under native purification conditions, an oligomeric form (M_r = 230 000) and monomeric form (37 000) of protein B23 were purified by affinity chromatography. Both forms were identified by Western blot immunoassay and ELISA. The molecular weight of the oligomeric form of protein B23 was estimated to be 230 000 with a Stoke's radius and a sedimentation coefficient of 51 Å and 10 S, respectively. The oligomer (230 kDa) of protein B23 was dissociated into monomers (37 kDa) by treatment with 7 M urea. Quantitation of the monomer by gel scanning densitometry indicated that the oligomeric form of protein B23 is a hexamer containing four α and two β monomers (37 kDa). A trace amount of nucleic acids (amounting to less than 3% of the total mass) was detected in the affinity-purified oligomers of protein B23. Protein B23 may be a structural element which is involved in ribosome transport or assembly in the nucleus.