Limited proteolysis of rat liver nucleolin by endogenous proteases: effects of polyamines and histones

Polyamine-regulated translation of spermidine/spermine-N1-acetyltransferase

Rapid synthesis of the polyamine catabolic enzyme spermidine/spermine-N(1)-acetyltransferase (SSAT) in response to increased polyamines is an important polyamine homeostatic mechanism. Indirect evidence has suggested that there is an important control mechanism involving the release of a translational repressor protein that allows the immediate initiation of SSAT protein synthesis without RNA transcription, maturation, or translocation. To identify a repressor protein, we used a mass spectroscopy-based RNA-protein interaction system and found six proteins that bind to the coding region of SSAT mRNA. Individual small interfering RNA (siRNA) experiments showed that nucleolin knockdown enhances SSAT translation. Nucleolin exists in several isoforms, and we report that the isoform that binds to SSAT mRNA undergoes autocatalysis in the presence of polyamines, a result suggesting that there is a negative feedback system that helps control the cellular content of polyamines. Preliminary molecular interaction data show that a nucleolin isoform binds to a 5' stem-loop of the coding region of SSAT mRNA. The glycine/arginine-rich C terminus of nucleolin is required for binding, and the four RNA recognition motif domains are included in the isoform that blocks SSAT translation. Understanding SSAT translational control mechanisms has the potential for the development of therapeutic strategies against cancer and obesity.

The Nucleolus Takes Control of Protein Trafficking Under Cellular Stress

The nucleolus is a highly dynamic nuclear substructure that was originally described as the site of ribosome biogenesis. The advent of proteomic analysis has now allowed the identification of over 4500 nucleolus associated proteins with only about 30% of them associated with ribogenesis (1). The great number of nucleolar proteins not associated with traditionally accepted nucleolar functions indicates a role for the nucleolus in other cellular functions such as mitosis, cell-cycle progression, cell proliferation and many forms of stress response including DNA repair (2). A number of recent reviews have addressed the pivotal role of the nucleolus in the cellular stress response (1, 3, 4). Here, we will focus on the role of Nucleolin and Nucleophosmin, two major components of the nucleolus, in response to genotoxic stress. Due to space constraint only a limited number of studies are cited. We thus apologize to all our colleagues whose works are not referenced here.

The carboxy-terminal fragment of nucleolin interacts with the nucleocapsid domain of retroviral gag proteins and inhibits virion assembly

A yeast two-hybrid screen for cellular proteins that interact with the murine leukemia virus (MuLV) Gag protein resulted in the identification of nucleolin, a host protein known to function in ribosome assembly. The interacting fusions contained the carboxy-terminal 212 amino acids of nucleolin [Nuc(212)]. The nucleocapsid (NC) portion of Gag was necessary and sufficient to mediate the binding to Nuc(212). The interaction of Gag with Nuc(212) could be demonstrated in vitro and was manifested in vivo by the NC-dependent incorporation of Nuc(212) inside MuLV virions. Overexpression of Nuc(212), but not full-length nucleolin, potently and specifically blocked MuLV virion assembly and/or release. A mutant of MuLV, selected to specifically disrupt the binding to Nuc(212), was found to be severely defective for virion assembly. This mutant harbors a single point mutation in capsid (CA) adjacent to the CA-NC junction, suggesting a role for this region in Moloney MuLV assembly. These experiments demonstrate that selection for proteins that bind assembly domain(s) can yield potent inhibitors of virion assembly. These experiments also raise the possibility that a nucleolin-Gag interaction may be involved in virion assembly.

Effects of polyamine levels on the degradation of short-lived and long-lived proteins in cultured L-132 human lung cells

Biogenic polyamines have important regulatory functions in various biological processes and it has also been suggested that they could modulate intracellular protein degradation. For an overall assessment of the role of polyamines in this process, we have investigated the effect that the decrease in intracellular polyamine levels caused by inhibitors of polyamine biosynthesis brings about on the degradation of the pools of short- and long-lived proteins in cultured L-132 human lung cells. Treatment of cells with 100 microM (2R,5R)-delta-methyl acetylenic putrescine (MAP), a potent enzyme-activated irreversible inhibitor of ornithine decarboxylase, or with 100 microM MAP plus 50 microM N-butyl 1,3-diaminopropane, a specific inhibitor of spermine synthase, caused a similar decrease (65-70% of control) in the total intracellular levels of polyamines, although they affected the concentrations of spermidine and spermine differently. The effect of the two treatments on protein degradation was essentially the same. In polyamine-depleted cells we observed an inhibition of degradation in long-lived proteins of 16% (P<0.05), with a significant increase in the half-life (t12) of this pool from 100.5 to 120.1 h. This was concomitant with an increase of 26% (P<0. 05) in degradation in short-lived proteins, with a significant decrease in the t12 of this pool from 0.85 to 0.67 h. Recovery of polyamine levels by the addition of 50 microM spermidine to polyamine-depleted cells resulted in a restoration of the degradation rates in both pools of proteins. The way(s) by which polyamines could modulate proteolysis are discussed.

The physical association of casein kinase 2 with nucleolin

CK2 (formerly called casein kinase 2) is a ubiquitous messenger-independent serine/threonine protein kinase implicated in cell growth and proliferation. To investigate the regulation and functions of this enzyme, experiments were carried out to search for CK2-interacting proteins. The methods employed included an overlay technique, co-purification, co-immunoprecipitation, and the use of glutathione S-transferase (GST) CK2 fusion proteins. By the CK2 overlay technique, one protein of 110 kDa was found to bind to CK2 with very high affinity. The binding was inhibited by CK2 effectors such as heparin, polyarginine, and histone H1, but was not affected by the CK2 substrate, casein. Protein p110 was also detected by co-immunoprecipitation using anti-CK2 antiserum, suggesting an in vivo association of this protein with CK2. Co-purification of p110 with CK2 from Sf-9 cells that overexpressed CK2 was also observed through sequential chromatographic steps. Using GST fusion proteins of CK2, the CK2-p110 interaction was investigated further and was found to occur primarily through CK2 alpha or alpha' subunits, but not the beta subunit. Protein p110 was purified from 3T3 L1 mouse fibroblast cell lines using a GST-CK2 affinity resin. Amino acid sequence analysis of peptides obtained from the protein indicated that it is the nuclear protein, nucleolin. Furthermore, p110 was recognized by anti-nucleolin antiserum. At present, the physiological significance of the strong interaction between CK2 and nucleolin, an excellent substrate for the enzyme, is not clear. However, this association may be important for regulating rDNA transcription.

Nuclear matrix isolated from plant cells

Residual nuclear matrices can be successfully obtained from isolated nuclei of different monocot and dicot plant species using either high ionic or low ionic extraction protocols. The protein composition of isolated nuclear matrices depends on the details of isolation protocols. They are stable and present in all cases, a tripartite organization with a lamina, nucleolar matrix, and internal matrix network, and also maintain some of the basic architectural features of intact nuclei. In situ preparations demonstrate the continuity between the nuclear matrix and the plant cytoskeleton. Two-dimensional separation of isolated plant nuclear matrix proteins reveals a heterogeneous polypeptide composition corresponding rather to a complex multicomponent matrix than to a simple nucleoskeletal structure. Immunological identification of some plant nuclear matrix components such as A and B type lamins, topoisomerase II, and some components of the transcription and splicing machineries, internal intermediate filament proteins, and also specific nucleolar proteins like fibrillarin and nucleolin, which associate to specific matrix domains, establish a model of organization for the plant nuclear matrix similar to that of other eukaryotes. Components of the transcription, processing, and DNA-anchoring complexes are associated with a very stable nucleoskeleton. The plant matrix-attached regions share structural and functional characteristics with those of insects, vertebrates, and yeast, and some of them are active in animal cells. In conclusion, the available data support the view that the plant nuclear matrix is basically similar in animal and plant systems, and has been evolutionarily conserved in eukaryotes.