Occurrence of phosphorylated forms of an acidic protein in the large ribosomal subunit of rat liver

Beta 2-adrenergic receptor stimulated, G protein-coupled receptor kinase 2 mediated, phosphorylation of ribosomal protein P2

G protein-coupled receptor kinases are well characterized for their ability to phosphorylate and desensitize G protein-coupled receptors (GPCRs). In addition to phosphorylating the beta2-adrenergic receptor (beta2AR) and other receptors, G protein-coupled receptor kinase 2 (GRK2) can also phosphorylate tubulin, a nonreceptor substrate. To identify novel nonreceptor substrates of GRK2, we used two-dimensional gel electrophoresis to find cellular proteins that were phosphorylated upon agonist-stimulation of the beta2AR in a GRK2-dependent manner. The ribosomal protein P2 was identified as an endogenous HEK-293 cell protein whose phosphorylation was increased following agonist stimulation of the beta2AR under conditions where tyrosine kinases, PKC and PKA, were inhibited. P2 along with its other family members, P0 and P1, constitutes a part of the elongation factor-binding site connected to the GTPase center in the 60S ribosomal subunit. Phosphorylation of P2 is known to regulate protein synthesis in vitro. Further, P2 and P1 are shown to be good in vitro substrates for GRK2 with K(M) values approximating 1 microM. The phosphorylation sites in GRK2-phosphorylated P2 are identified (S102 and S105) and are identical to the sites known to regulate P2 activity. When the 60S subunit deprived of endogenous P1 and P2 is reconstituted with GRK2-phosphorylated P2 and unphosphorylated P1, translational activity is greatly enhanced. These findings suggest a previously unrecognized relationship between GPCR activation and the translational control of gene expression mediated by GRK2 activation and P2 phosphorylation and represent a potential novel signaling pathway responsible for P2 phosphorylation in mammals.

Ribosome and protein synthesis modifications after infection of human epidermoid carcinoma cells with herpes simplex virus type 1

Modifications of ribosomes have been investigated in human epidermoid carcinoma-2 cells at different stages of herpes simplex virus type 1 infection. Very early in infection, there is an increase in ribosomal protein S6 phosphorylation even in the absence of serum. The same result is obtained in the presence of actinomycin D. At early infection time, ribosomal proteins S2, S3a and Sa are newly phosphorylated. At early and early-late times, three phosphorylated non-ribosomal proteins (v1, v2 and v3) are differently associated temporally to ribosomes. Analyses of proteins extracted from 40S subunits, 80S ribosomes and polysomes show that v1 and v2 are distributed differently among the different ribosomal populations. S6 phosphopeptides were found to be identical after serum stimulation and after viral infection. In every case phosphoserine and phosphothreonine were identified in S6. Only phosphoserine was found in other phosphorylated proteins. Our results indicate that herpes simplex virus type 1 is able to modify pre-existing ribosomes: (i) by stimulating a pre-existing kinase for S6 phosphorylation even in the absence of serum and of viral genome expression; (ii) by inducing new specific kinase activity(ies); and (iii) by association of new, phosphorylated proteins to ribosomes. These ribosomal modifications are correlated with changes in protein synthesis, as shown by two-dimensional electrophoretic analyses of newly synthesized 35S-labelled proteins.

Reconstitution of the active rat liver 60 S ribosomal subunit from different preparations of core particles and split proteins

Proteins extracted from the 60 S rat liver ribosomal subunit with 50% ethanol/0.5 M K Cl produced only a partial reactivation of the corresponding core particles. In contrast, the same split proteins were able to reactivate the core particles prepared with dimethyl-maleic anhydride (DMMA) to the same level as that observed using the DMMA-split proteins, i.e. 60-80% of the control according to the catalytic activities tested. Comparative analysis of the two split protein fractions showed only four common proteins: P1-P2, which alone restored part of the activities, especially the EF-2-dependent GTPase one, and L10a, L12, which must be responsible for the additional reactivation. The poor ability of the ethanol/KCl core particles to be reactivated was shown to be probably related to a conformational alteration which destabilized the 5 S RNA-protein complex. Proteins present in the ethanol/KCl wash of Saccharomyces cerevisiae 60 S subunits were found to be partly active in subunit reconstitution using rat liver DMMA core particles.

5S-rRNA-containing ribonucleoproteins from rabbit muscle and liver. Complex and partial primary structures

A 5S-rRNA-containing ribonucleoprotein was purified to homogeneity from a rabbit muscle extract through its affinity to phosphofructokinase-1 and then structurally characterized. This RNP was compared to the 5S-rRNA-containing ribonucleoprotein extracted from rabbit liver ribosomal 60S subunits with EDTA. Analytical gel filtration revealed a molecular mass of 70-80 kDa for both complexes. Gel electrophoresis of the ribosomal complex revealed three protein components, one migrating as a band of 35 kDa and two other small polypeptides of apparently 16.5 kDa and 17.5 kDa. In the sarcoplasmic RNP these small polypeptides were absent. However, besides a major component of 35 kDa, up to five slightly larger and smaller species of 31.5-36.5 kDa were detected. Despite this heterogeneity, only one N-terminal amino acid sequence was obtained for the isolated sarcoplasmic protein, suggesting a C-terminal heterogeneity of one single polypeptide. Within the first 46 amino acid residues no difference between the sequences of the isolated 35-kDa components of sarcoplasmic and ribosomal complexes was found. Homology criteria indicated that this component belongs to the ribosomal protein L5 family. The RNA was identified by complete enzymatic sequencing as 5S rRNA; it was also identical in both complexes and is strongly homologous to 5S rRNA of man. Both L5-5S-RNA complexes could be resolved by hydroxyapatite chromatography into three species still consisting of both protein and RNA. 5'-Terminal dephosphorylation experiments showed that this heterogeneity is exclusively due to the differing number (1-3) of 5'-terminal phosphates. The two additional low-molecular-mass proteins were stably associated to the ribosomal RNP at high salt concentrations in a stoichiometry of about 2:1. They were identified as the acidic phosphoproteins P2/P3 by N-terminal sequencing. High phosphate concentrations facilitated their dissociation from the L5-5S-RNA complex. For the sarcoplasmic L5-5S-RNA complex a hitherto unknown interaction with phosphofructokinase-1, affecting the enzymatic properties, was demonstrated.

Role of acidic phosphoproteins in the partial reconstitution of the active 60 S ribosomal subunit

We have recently shown that rat liver 60 S ribosomal subunits active in protein synthesis can be reconstituted from inactive core particles lacking 30% of the total proteins, mainly L10a, L12, L22, L24, A33 and the acidic phosphoproteins P1-P2, obtained by treatment of 60 S subunits with dimethylmaleic anhydride [(1987) Eur. J. Biochem. 163, 15-20]. In this study, an ethanol extract of the 60 S subunit which contains only P1 P2 was also shown to be effective in reconstitution with the DMMA-core-particles: it strongly stimulated the EF-2-dependent GTP hydrolysis and, to a lesser extent, polyphenylalanine synthesis; like the DMMA wash it shifted the thermal denaturation curve of the DMMA-core particles towards that of control subunits. Prior dephosphorylation of the ethanol extract by alkaline phosphatase inhibited the reconstruction process.

Ribosomal protein phosphorylation in vivo and in vitro by vaccinia virus

Ribosomal protein phosphorylation was investigated in Ehrlich ascites tumor cells infected with vaccinia virus (Copenhagen strain). After 90 min of simultaneous infection and 32P-labelling, ribosomal proteins Sa, S2 and S13 appear specifically phosphorylated as well as Sb/La, P1 and S6, which are also phosphorylated in control cells. Sa is an acidic protein, whose phosphorylation has not been observed previously. A kinetic study showed that S2 is phosphorylated very rapidly within 10 min after the beginning of infection and it is complete 1 h later. The phosphorylation of S13 begins after a lag time of about 1 h and is completed after about 2.5 h of infection. Moreover only one phosphate is incorporated into S13 on a serine residue while up to four phosphates are incorporated into S2, the first on a serine and the three following on threonine residues. In vivo experiments, carried out in the presence of cycloheximide and cordycepin, suggest a viral origin for the kinase involved in the phosphorylation of S2 and S13. Moreover, in vitro experiments demonstrated that the kinase associated with the viral cores is capable of phosphorylating S2 on a serine residue only. In our cell/virus system, no significant difference in S6 phosphorylation was detected, when compared to uninfected cells. It is concluded that the specific and efficient phosphorylation of three ribosomal proteins from the 40S ribosomal subunit correlate well with possible translational mechanisms ensuring the efficient expression of early and late genes of vaccinia virus. In the light of these and previous results [Person, A. and Beaud, G. (1986) J. Biol. Chem. 261, 8283-8289], a mechanism is proposed for the shut-off of host protein synthesis and the selective translation of mRNAs of viral origin.

Characterization and properties of a 5S-RNA-protein complex released from heated 60S ribosomal subunits

When rat liver 60S ribosomal subunits were heated in phosphate buffer in the presence of MgCl2, 5S RNA was released in the form of a nucleoprotein complex (RNPH), which was isolated either by electrophoresis in polyacrylamide gel or centrifugation through a sucrose gradient. In addition to L5 several proteins of functional significance were identified in the complex: the acidic phosphoproteins P1-P2 and, as weaker spots, L3-L4, L6-L7 and L22. Most of these proteins were also found, but only as traces, in the RNPEDTA used as a control. RNPH was able to associate with 40S subunits. Our results support the interpretation that RNPH is located at the subunits' interface, at or near the peptidyl-transferase center.

Turnover rates of phosphoryl groups in ribosomal proteins of Physarum polycephalum. Evidence for two different mechanisms

The rate of phosphate exchange in individual ribosomal proteins of Physarum polycephalum was determined in vivo. It was observed that the phosphoryl groups of S3, the major phosphoprotein, had a turnover rate of 1.5% per minute. The phosphoryl groups of proteins L1, L20 and L24 were stable. These results show that the phosphorylation of ribosomal proteins is regulated by at least two different mechanisms. The rapid turnover of phosphoryl groups of the major phosphoprotein is in agreement with the general observation that the phosphate content of this protein is modulated by the physiological state of the cells and possibly involved in the regulation of ribosome activity. The absence of phosphate exchange in acidic proteins suggests that these groups could play a structural role in the ribosome functions.

Differences in electrophoretic behaviour of eight ribosomal proteins from rat and rabbit tissues and evidence for proteolytic action on liver proteins

Divergence exists between eight ribosomal proteins of rat and rabbit. This is the first time such a divergence has been precisely demonstrated among mammals. In addition, a proteolytic activity, giving the appearance of modified proteins, is observed in the liver of both species but not in rabbit reticulocytes. These results were made possible by a recently developed method. Ribosomal proteins were compared by two-dimensional polyacrylamide gel electrophoresis in four different but related systems. The precise position of each individual protein was established in each of the four systems. Certain ribosomal proteins from different tissues, seemingly identical in one system were found to be different in other systems. Significant differences occurred between proteins of each ribosomal subunit from the two different species. Variation between reticulocyte and liver ribosomal proteins of the rabbit were minor. Several liver proteins of both species change position or disappear and apparently new proteins appear, if appropriate steps are not taken to prevent proteolysis. Differences in behavior of three small subunit and five large subunit proteins are attributable to an evolutionary divergence between the two species.

Spot position of rat liver ribosomal proteins by four different two-dimensional electrophoreses in polyacrylamide gel

Separation of the proteins from rat liver 40S and 60S ribosomal subunits and polysomes was done in four different two-dimensional polyacrylamide gel electrophoresis systems. The first dimension was run at acidic or basic pH, the second dimension either with sodium dodecyl sulphate or at acidic pH in 18% acrylamide. The position of each individual protein of both subunits and polysomes was determined in each system. This identification resulted from a new method avoiding any pervious purification of individual proteins. The new "proposed uniform nomenclature for mammalian ribosomal proteins" (McConkey et al. in press) was used for numbering the proteins in the four systems.