Nucleolin, the Major Nucleolar Protein of Growing Eukaryotic Cells: an Unusual Protein Structure Revealed by the Nucleotide Sequence

Nucleolin (also called C23) is the major nucleolar protein of exponentially growing eukaryotic cells. It is found associated with intranucleolar chromatin and preribosomal particles. Through use of a polyclonal antiserum, nucleolin cDNA clones were isolated from a Chinese hamster ovary cell library constructed in the expression vector lambda gt11. The isolated cDNAs encoded a polypeptide containing 679 residues of the 713 amino acids of nucleolin. The amino acid sequence presents several unusual features: in particular, repetitive sequences are found at both ends of the molecule. A repeat, Hy-Thr-Pro-Hy-Lys-Lys-Hy-Hy, in which Hy is a nonpolar residue, is found six times in the NH2-end proximal portion, followed by three acidic stretches containing 25, 25, and 33 glutamic acid or aspartic acid residues. Four potential phosphorylation sites (serines) are also observed in this region. The COOH-terminal proximal portion of the protein carries a glycine-rich region with fairly regularly interspersed phenylalanine and dimethylarginine residues. The two terminal portions of the molecule exhibit unique potential secondary structures: alpha-helix (NH2 terminus) and extended (COOH terminus). The central region exhibits alternating hydrophobic and hydrophilic stretches. Five potential N glycosylation sites are detected. The structure of this protein may reflect two functions in preribosome biogenesis: interaction with chromatin (NH2 terminus) and with preribosomes (COOH terminus).