Phosphorylation of proteins of ribosomes and nucleolar preribosomal particles from Novikoff hepatmoa ascites cells

Deciphering the human nucleolar proteome

Nucleoli are plurifunctional nuclear domains involved in the regulation of several major cellular processes such as ribosome biogenesis, the biogenesis of non-ribosomal ribonucleoprotein complexes, cell cycle, and cellular aging. Until recently, the protein content of nucleoli was poorly described. Several proteomic analyses have been undertaken to discover the molecular bases of the biological roles fulfilled by nucleoli. These studies have led to the identification of more than 700 proteins. Extensive bibliographic and bioinformatic analyses allowed the classification of the identified proteins into functional groups and suggested potential functions of 150 human proteins previously uncharacterized. The combination of improvements in mass spectrometry technologies, the characterization of protein complexes, and data mining will assist in furthering our understanding of the role of nucleoli in different physiological and pathological cell states.

Conventional and nonconventional roles of the nucleolus

As the most prominent of subnuclear structures, the nucleolus has a well-established role in ribosomal subunit assembly. Additional nucleolar functions, not related to ribosome biogenesis, have been discovered within the last decade. Built around multiple copies of the genes for preribosomal RNA (rDNA), nucleolar structure is largely dependent on the process of ribosome assembly. The nucleolus is disassembled during mitosis at which time preribosomal RNA transcription and processing are suppressed; it is reassembled at the end of mitosis in part from components preserved from the previous cell cycle. Expression of preribosomal RNA (pre-rRNA) is regulated by the silencing of individual rDNA genes via alterations in chromatin structure or by controlling RNA polymerase I initiation complex formation. Preribosomal RNA processing and posttranscriptional modifications are guided by a multitude of small nucleolar RNAs. Nearly completed ribosomal subunits are exported to the cytoplasm by an established nuclear export system with the aid of specialized adapter molecules. Some preribosomal and nucleolar components are transiently localized in Cajal bodies, presumably for modification or assembly. The nonconventional functions of nucleolus include roles in viral infections, nuclear export, sequestration of regulatory molecules, modification of small RNAs, RNP assembly, and control of aging, although some of these functions are not well established. Additional progress in defining the mechanisms of each step in ribosome biogenesis as well as clarification of the precise role of the nucleolus in nonconventional activities is expected in the next decade.

The major phosphorylation site of nucleophosmin (B23) is phosphorylated by a nuclear kinase II

Nucleophosmin (B23) was phosphorylated in vitro with [gamma-32P]ATP and a nuclear kinase (type II) purified from HeLa cells. The phosphorylation was inhibited by heparin and by 2,3-diphosphoglycerate. Peptide mapping analysis indicated that the phosphorylation site in vitro was identical to that in vivo. Purified nucleoli have a similar kinase that phosphorylated nucleophosmin at the same site. These results indicated that nucleophosmin is phosphorylated in vivo by a nucleolar kinase (type II).

Interactions of minute virus of mice and adenovirus with host nucleoli

Biochemical evidence is presented that both minute virus of mice (MVM) and adenovirus interact with the nucleolus during lytic growth and that MVM can also target specific changes involving nucleolar components in adenovirus-infected cells. These virus-nucleolus interactions were studied by analysis of intranuclear compartmentalization of both viral DNAs and host nucleolar proteins: (i) MVM in mouse cells (its normal host) replicates its DNA in the host nucleoli; (ii) specific nucleolar proteins as well as small nuclear ribonucleoprotein antigens are recompartmentalized to multiple intranuclear foci in adenovirus-infected HeLa cells; and (iii) when adenovirus helps MVM DNA replication in a nonpermissive human cell (HeLa), the MVM DNA is also recompartmentalized for synthesis. The data suggest mechanisms for disruption of nucleolar function common to oncogenic or oncolytic virus lytic growth and cell transformation.

Association of protein C23 with rapidly labeled nucleolar RNA

The association of nucleolar phosphoprotein C23 with preribosomal ribonucleoprotein (RNP) particles was examined in Novikoff hepatoma nucleoli. RNA was labeled with [3H]uridine for various times in cell suspensions, and RNP particles were extracted from isolated nucleoli and fractionated by sucrose gradient ultracentrifugation. The majority of protein C23 cosedimented with fractions containing rapidly labeled RNA (RL fraction). To determine whether there was a direct association of RNA with protein C23, the RL fraction was exposed to ultraviolet (UV) light (254 nm) for short periods of time. After 2 min of exposure there was a 50% decrease in C23 as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analyses, with no significant further decrease at longer times. When UV-treated fractions were subjected to phenol/chloroform extractions, as much as 30% of the labeled RNA was found in the phenol (protein) layer, indicating that RNA became cross-linked to protein. Similarly, there was an increase in protein C23 extracted into the water layer after irradiation. By SDS-PAGE analyses the cross-linked species migrated more slowly than protein C23, appearing as a smear detected either by [3H]uridine radioactivity or by anti-C23 antibody. With anti-C23 antibodies, up to 25% of the labeled RNA was precipitated from the RL fraction. Dot-blot hybridizations, using cloned rDNA fragments as probes, indicated that the RNA in the RL fraction and the immunoprecipitated RNA contained sequences from 18S and 28S ribosomal RNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of nucleolar phosphoproteins B23 and C23 during mitosis

Nucleolar phosphoproteins B23 and C23 were simultaneously localized in unsynchronized male rat-kangaroo PtK2 cells during mitosis using a mouse monoclonal antibody against protein B23 and a rabbit antibody against protein C23. The distribution of proteins B23 and C23 during mitosis was compared with the distribution of the silver staining protein. During interphase, proteins B23 and C23 were both localized to the nucleolus. As the nucleolus disappeared in prophase, the distribution of protein B23 became nucleoplasmic, whereas most of protein C23 remained associated with the disappearing nucleolus. Throughout metaphase and anaphase protein B23 was found associated with the chromosomes, whereas protein C23 seemed to disappear. When the nucleolus reformed during telophase, protein C23 appeared first in 'prenucleolar bodies' and then in the nucleolus, whereas protein B23 did not appear in the nucleolus until late telophase or early G1 phase. Silver staining during mitosis closely paralleled the distribution of protein C23, supporting previous conclusions that protein C23 is a silver staining nucleolus organizer region (NOR) protein [19, 20].

Sequence of a phosphorylation site in nucleolar protein B23

The sites of phosphorylation in protein B23, a silver-staining preribosomal ribonucleoprotein particle protein, were analyzed by tryptic peptide mapping. Three 32P peptides were found using in vitro labeling of nucleoli. An additional unlabeled phosphopeptide was identified by amino acid analysis. The sequence of the latter was Asp-Thr(P)-Pro-Ala-Lys. These results suggest that protein B23 contains one class of site labeled rapidly in vitro and another type of site phosphorylated only in vivo.

Nucleolar phosphoprotein phosphatase from Novikoff hepatoma and rat liver: characterization and partial purification

Phosphoprotein phosphatase which dephosphorylates 32P-labeled nucleolar protein substrates was found in nucleoli of Novikoff hepatoma ascites cells and normal rat liver. The activity was extracted in high yield from nucleoli with 0.01 M Bis/Tris (pH 7.0). Low ionic strength was also required for activity: the activity was only 50% of maximum in 0.075 M NaCl. Activity was affected differently by various divalent cations: MgCl2 had little effect: CaCl2, MnCl2 and CoCl2 above 4 mM inhibited the activity 30--60%; ZnCl2 above 2 mM completely destroyed the activity. EDTA had no effect, indicating that divalent cations are probably not required. The enzyme activity was enhanced 20% by 5--8 mM dithiothreitol and was inhibited 60% by 7--10 mM N-ethylmaleimide indicating a requirement for free sulfhydryl groups. The Km of the extracted enzyme for 32P-labeled nucleolar protein was 0.6 mg/ml. The phosphatase was capable of dephosporylating the major phosphorylated nucleolar proteins C23-24 and B23-24 and also histone H1. The enzyme was purified more than 200-fold on hydroxyapatite followed by DEAE-Sephadex, which resolved the activity into three major components. The activity of enzyme extracted from Novikoff hepatoma nucleoli was approximately 2.5 times greater than from normal liver nucleoli.

Increased activity of rat liver nucleolar protein kinase following triiodothyronine administration

Triiodothyronine (T3) administration to thyroidectomized rats induces a significant increase in the nucleolus-associated protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) activity. The general properties of the protein kinase solubilized from liver nucleoli have been investigated. Mg2+ (20 mM) is essential for the reaction and an appropriate concentration of NaCl (100 mM) is required to achieve maximal phosphorylation rates. The optimal pH for casein phosphorylation is 7.6. The kinase phosphorylates casein more efficiently than phosvitin and displays an almost undetectable activity towards histones and protamine. No significant stimulation of the kinase activity by cyclic AMP has been detected. The apparent Km values for casein and ATP are 1.5 mg/ml and 1.5-10(-5) M, respectively, and are not affected by the hormone administration.

Increased in vitro phosphorylation of rat liver nucleolar proteins following triiodothyronine administration

It has been shown that triiodothyronine (Ta) administration to thyroidectomized rats induces an increase in the in vitro net 32P uptake into liver nucleolar proteins. Such an increase depends on a stimulation of the nucleolus-associated protein kinase activity and not on a lower dephosphorylation rate.

Differences between newly formed and recycled free small ribosome subunits in liver cytoplasm

Heterogeneity among the free small ribosomal subunits of rat liver was studied. Newly made free small subunits differ from the bulk of the free subunits in the following ways. (1) The newly made free small subunits sediment slightly faster than most free small subunits and form a band at slightly lower densities. (2) Mature free small subunits, prepared under conditions which minimize the amount of loosely bound protein, readily dimerize in solutions of low ionic strength. Little dimerization is found with newly formed subunits. This permits the separation of fractions enriched in newly formed subunits. (3) The fraction enriched in newly formed subunits contains a distinctive additional protein as compared the most of the free small subunits.

In vivo and in vitro phosphorylation of ribosomal proteins by protein kinases from Saccharomyces cerevisiae

From the high salt wash of the ribosomes of the yeast Saccharomyces cerevisiae, three protein kinases have been isolated and separated by DEAE-cellulose chromatography. The three kinases differ in their abilities to phosphorylate substrates such as histones (calf thymus), casein, and S. cerevisiae ribosomes; two of the kinases showed increased activity in the presence of cyclic adenosine 3',5'-monophosphate when histones and 40S ribosomal subunits were used as substrates. The protein kinases catalyzed phosphorylation of certain proteins of the 40S and 60S ribosomal subunits, and 80S ribosomes in vitro. Nine proteins of the 80S ribosome, seven proteins of the 40S subunit, and eleven of the 60S subunit were phosphorylated; different proteins were modified to various extents when different kinases were used. We have identified several proteins of 40S and 60S ribosomal subunits which are not available to the kinases in the 80S particles. Ribosomes isolated from S. cerevisiae cells growing in logarithmic phase of growth were found to contain a number of phosphorylated proteins. Studies by two-dimensional polyacrylamide gel electrophoresis indicated that the ribosomal proteins phosphorylated in vivo correspond with those phosphorylated in vitro. The relationship of in vivo phsophorylation of ribosomes to the growth and physiology of S. cerevisiae is not known.

Phosphorylation of ribosome-associated proteins during the mammalian cell cycle. Unique phosphorylation of a specific protein during mitosis

Chinese hamster ovary cells in monolayer culture were incubated with [32P] phosphate. Ribosome-associated proteins, including both structural proteins and those tightly bound to washed, centrifuged ribosomes, were isolated and separated by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. The electrophoretic pattern showed five major regions or peaks of 32P radioactivity which represented phosphorylated ribosome-associated proteins with molecular weights of 17 500, 23 000, 30 000, 38 000, and 57 000. When asynchronous cells were pulse-labeled with [32P] phosphate, the predominant peak of 32P radioactivity was associated with the protein of 38 000 daltons. Similar results were obtained with cells synchronized in the G1, S, or G2 phase of the mammalian cell cycle. Conversely, proteins isolated from the ribosomes of mitotic cells, collected and labeled with [32P] phosphate in the presence of colcemid, showed a new and predominant peak of 32P radioactivity migrating with a protein of 45 000 daltons. When cells labeled in mitosis were allowed to progress into G1 phase, this peak of 32P radioactivity rapidly disappeared from the electrophoretic pattern. These results suggest that a specific protein associated with the ribosomes was phosphorylated uniquely during the mitotic phase of the cell cycle.