Phosphorylation of nucleolin by a nucleolar type NII protein kinase

Abstract

Nucleolin [C23 or 100 kilodaltons (kDa)] is the major nucleolar phosphorylated protein in exponentially growing Chinese hamster ovary cells. A nucleolar cyclic nucleotide independent protein kinase copurified with nucleolin in a complex which could be dissociated by hydroxyapatite chromatography. The kinase was stimulated by spermine and inhibited by heparin and presented most of the properties of nuclear casein kinase NII. Kinetic analyses showed the apparent Km value for nucleolin (7 X 10(-4) mg/mL) to be lower than those for other casein kinase II substrates such as nuclear protein HMG 14 (0.15 mg/mL), topoisomerase I (0.025 mg/mL), or topoisomerase II (0.04 mg/mL). Similarly, Vmax values were higher for nucleolin than for other substrates. Nucleolin thus appears to be a natural preferential substrate of nucleolar casein kinase NII. The kinase phosphorylated nucleolin in vitro at serine residues in a 29-kDa CNBr fragment located near the amino terminus of the molecule. The enzyme labeled typical casein kinase II sites. These sites were found predominantly in two highly acidic tryptic fragments designated A (residues 21-49) and C (residues 180-221) which contained serines having at least two acidic residues on their carboxyl-terminal sides. These results demonstrate the existence in the nucleolus of a type of NII protein kinase that uses a protein involved in ribosome assembly as preferential substrate.

Fibroblast Growth Factor-2 Binds to the Regulatory β Subunit of CK2 and Directly Stimulates CK2 Activity toward Nucleolin

“…These findings support the hypothesis that CK2 could be a target of FGF-2 involved in nuclear functions of the growth factor. We reported previously that in nuclei isolated from ABAE confluent cells, CK2 activity is inadequate to phosphorylate endogenous nucleolin, a major substrate for CK2 (31). The addition of exogenous CK2 to such nuclei induced not only protein phosphorylation but also ribosomal genes transcription (23).…”

“…CK2 and nucleolin was purified from CHO cells (Mons University, Hainaut, Belgium). Nuclei were isolated and lysed in a low ionic strength buffer and the resulting S2 extract was applied to an heparin-Sepharose column (Pharmacia Biotech Inc.) (31). Fractions containing nucleolin and CK2 were then subjected to anionexchange chromatography (Mono Q, Pharmacia).…”

Mechanisms of disruption of meristematic competence by microgravity inArabidopsisseedlings

“…33 Independent of this auxin-related function, CK2 is known to play a major role in the regulation of the cell cycle at different levels, specifically in the G2/M transition 34 and also in ribosome biogenesis, since phosphorylation of nucleolin by CK2 is necessary for a normal processing of pre-ribosomal precursors. 35,36 Therefore, although there is no direct evidence of the involvement of CK2 in the response to gravity alteration, the known functions of this essential protein kinase appoint Figure 2. Schematic model of the main factors and functional processes playing a role in the regulation of the functionality of meristematic cells by environmental gravity.…”

Immunocytochemical localization of casein kinase II during interphase and mitosis.

“…Nucleolus-associated CKII has been identified, has been shown to phosphorylate nucleolin as a substrate, and may be involved in ribosome assembly (Caizergues-Ferrer et al ., 1987 ;Schneider and Issinger, 1988) and regulation of rDNA transcription (Belenguer et al, 1989).…”