Characterization of a 48‐kDa nucleic‐acid‐binding fragment of nucleolin

Nucleolin (C23 or 100 kDa) is an abundant single‐stranded‐nucleic‐acid‐binding nucleolar protein proposed to be involved in the early stages of ribosome assembly. A stable 48‐kDa fragment of the protein was produced either by proteolytic activity present in nucleolar extracts or by added trypsin. The hydrodynamic and DNA‐binding properties of the 48‐kDa fragment were compared with the parent molecule. Protein sequencing indicated that the fragment begins at residue 282; amino acid composition of the fragment including 10 – 12 methylated arginine residues suggested that the fragment contains the entire COOH‐terminal two‐thirds of the protein. The 48‐kDa fragment was more globular than nucleolin, as indicated by a lower frictional coefficient (1.3 vs. 2.0 for nucleolin) and a similar sedimentation coefficient (4.1 – 4.3S) in spite of the reduction in molecular mass. Although the 48‐kDa fragment retained single‐stranded‐DNA‐binding activity, the binding capacity and the ability to reassociate DNA were about fivefold and sixfold lower, respectively, than nucleolin. Similarly, tenfold higher concentrations of the 48‐kDa fragment were required to form nucleoprotein aggregates. These results suggest that nucleolin contains a globular COOH‐terminal domain for nucleic‐acid binding and a NH2‐terminal region which is involved in protein‐protein interactions and modulating nucleic‐acid‐binding activity.