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The native 80S ribosomes isolated from Saccharomyces cerevisiae (strain W303) cells was phosphorylated by two endogenous protein kinases: multifunctional casein kinase-2 (CK-2) and specific 60S kinase. Three acidic proteins within the 60S ribosomal subunit: YP1(3, YPlp' and YP2a are phosphorylated by both kinases. The other two proteins: YPla and YP2(3 are predominantly phosphorylated by CK-2 but not by 60S kinase. This was confirmed in the experiment with the recombinant protein, YP2P, as a substrate, which is practically not phosphorylated by specific 60S kinase. These results together with the previous data based on the target amino-acid se­quences suggest that, in addition to the multifunctional casein kinase-2 and specific 60S kinase, there exist probably other protein kinase(s) which phosphorylate the ribosomal acidic proteins in the cell.
Several halogeno benzimidazoles and 2-azabenzimidazoles, previously shown to be relatively selective inhibitors of protein kinases CK-I and/or CK-II from various sources, including CK-II from yeast [Szyszka et al. (1995) Biochem. Biophys. Res. Commun. 208, 418-4241 inhibit also the yeast ribosomal protein kinase PK60S. The most effective inhibitor of CK-II and PK60S was tetrabromo-2-azabenzimidazole (TetraBr-2-azaBz), which was competitive with respect to ATP (and GTP in the case of CK-II) with Ki values of 0.7 uM for CK-II, and 0.1 pM for PK60S. PK60S phosphorylates only three (YPlp, YP1(J', YP2a) out of five polypeptides of ppl3 kDa acidic proteins of 60S subunit phosphorylated by CK-II [Szyszka et al. (1995) Acta Biochim. Polon. 42, 357-3621. Accordingly, TetraBr-2-azaBz inhibits phosphorylation only of these polypeptides, catalysed by PK60S. Addition of TetraBr-2-azaBz to cultures of yeast cells, at concentrations which were without effect on cell growth, led to inhibition of intracellular phosphorylation of ribosomal acidic proteins, paralleling that observed in vitro. TetraBr-2-azaBz is shown to be a useful tool for studies on the intracellular regulation of phosphorylation of the ribosomal 60S acidic proteins, which are involved in formation of active ribosomes.
A gene for ribosomal protein P0 that belongs to the family of ribosomal P proteins was isolated from a Neurospora crassa cDNA library, using polyclonal antibodies against recombinant P0 protein from Saccharomyces cerevisiae. This is the first gene for ribosomal P0 protein to be cloned from filamentous fungi. The derived P0 protein sequence has a strong homology to other eukaryotic P0 proteins; yet, there is a nota­ble alteration in the conservative C-terminal region, placing this protein among the unique sequences from protozoan parasites.
The phosphorylation sites of ribosomal acidic proteins (P proteins) from Saccharomyces cerevisiae were studied in vivo and in vitro by using CK-2, PK60S and RAP protein kinases. The three enzymes phosphorylate the last serine residues located in a highly conserved carboxyl end of the polypeptide chains. This was established by two-dimensional analysis of tryptic phosphopeptides from 32P-labelled proteins YP1 alpha, YP1 beta, YP2 alpha and YP2 beta, and by kinetic studies of the protein kinases with synthetic peptides corresponding to the fragments of endogenous ribosomal acidic polypeptides. In experiments with both endogenous P proteins and synthetic peptides as substrates protein kinase PK60S demonstrated unusual substrate specificity. In contrast to CK-2 and RAP protein kinases, PK60S phosphorylates predominantly two of the four P proteins, YP1 alpha and YP2 beta, with kinetic constants dependent on the primary structure of the N-terminal region of the polypeptide containing the target residue. The neutral amino acid, alanine, at position 3 in the peptide AAEESDDD (polypeptide fragments of YP1 beta and YP2 alpha) decreases the K(m) value more than 10-fold by comparison with the basic lysine residue at the same position in the peptide AKEESDDD (polypeptide fragments of YP1 alpha and YP2 beta).
The yeast acidic ribosomal P-proteins YP1α, YP1β, YP2a and YP2b were studied for a possible transactivation potential beside their ribosomal function. The fusions of P-proteins with the GAL4 DNA-binding domain were assayed toward their transcriptional activity with the aid of reporter genes in yeast. Two of the P-proteins, YP1α and YP1β, exhibited transactivation potential, however, only YP1α can be regarded as a potent transactivator. This protein was able to transactivate a reporter gene associated with two distinct promoter systems, GAL1 or CYC1. Additionally, truncated proteins of YP1α and YP1β were analyzed. The N-terminal part of YP1α fused to GAL4-BD showed transactivation potential but the C-terminal part did not. Our results suggest a putative extraribosomal function for these ribosomal proteins which consequently may be classified as "moonlighting" proteins.
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