Partial reassembly of active 60S ribosomal subunits from rat liver following treatment with dimethylmaleic anhydride

Cancer-mutated ribosome protein L22 (RPL22/eL22) suppresses cancer cell survival by blocking p53-MDM2 circuit

Several ribosomal proteins (RPs) in response to various ribosomal stressors have been shown to play a critical role in p53-dependent regulation of cell cycle arrest, apoptosis and tumor suppression. Here, we report ribosomal protein L22 (RPL22/eL22) as a novel p53 activator highly mutated (mostly deletion mutation) in various types of human cancers, but not essential for ribosomal biogenesis in normal cells. Ectopic expression of RPL22/eL22 suppressed the colony formation of cancer cells in a p53-dependent manner, whereas knockdown of RPL22/eL22 significantly compromised p53 activation by Actinomycin D, rescuing p53-induced G1/G0 cell cycle arrest. Interestingly, human tumors with RPL22/eL22 deletion appeared to sustain wild type p53. Mechanistically, RPL22/eL22 bound to MDM2 acidic domain and inhibited MDM2-mediated p53 ubiquitination and degradation, hence extending the half-life of p53. Ribosome-profiling analysis revealed that induction of ribosomal stress by Actinomycin D leads to the increase of ribosome-free RPL22/eL22 pool. Also, RPL22/eL22 formed a complex with MDM2/RPL5/uL18/RPL11/uL5 and synergized with RPL11/uL5 to activate p53. Furthermore, the N terminus of RPL22/eL22 bound to MDM2, while the C terminus interacted with RPL5/uL18/RPL11/uL5; both of these two fragments activated p53 by inhibiting MDM2. Our study indicates that RPL22/eL22 highly mutated in human cancers plays an anti-cancer role likely through regulation of the MDM2-p53 feedback loop, and also suggests that targeting the RPL22/eL22-MDM2-p53 pathway could be a potential strategy for future development of anti-cancer therapy.

Exploring human 40S ribosomal proteins binding to the 18S rRNA fragment containing major 3'-terminal domain

Association of ribosomal proteins with rRNA during assembly of ribosomal subunits is an intricate process, which is strictly regulated in vivo. As for the assembly in vitro, it was reported so far only for prokaryotic subunits. Bacterial ribosomal proteins are capable of selective binding to 16S rRNA as well as to its separate morphological domains. In this work, we explored binding of total protein of human 40S ribosomal subunit to the RNA transcript corresponding to the major 3'-domain of 18S rRNA. We showed that the resulting ribonucleoprotein particles contained almost all of the expected ribosomal proteins, whose binding sites are located in this 18S rRNA domain in the 40S subunit, together with several nonspecific proteins. The binding in solution was accompanied with aggregation of the RNA-protein complexes. Ribosomal proteins bound to the RNA transcript protected from chemical modification mostly those 18S rRNA nucleotides that are known to be involved in binding with the proteins in the 40S subunit and thereby demonstrated their ability to selectively bind to the rRNA in vitro. The possible implication of unstructured extensions of eukaryotic ribosomal proteins in their nonspecific binding with rRNA and in subsequent aggregation of the resulting complexes is discussed.

Acquisition of a stable structure by yeast ribosomal P0 protein requires binding of P1A–P2B complex: In vitro formation of the stalk structure

Saccharomyces cerevisiae ribosomal stalk consists of five proteins: P0 protein, with molecular mass of 34 kDa, and four small, 11 kDa, P1A, P1B, P2A and P2B acidic proteins, which form a pentameric complex P0-(P1A-P2B)/(P1B-P2A). This structure binds to a region of 26S rRNA termed GTPase-associated domain and plays a crucial role in protein synthesis. The consecutive steps leading to the formation of the stalk structure have not been fully elucidated and the function of individual P-proteins in the assembling of the stalk and protein synthesis still remains elusive. We applied an integrated approach in order to examine all the P-proteins with respect to stalk assembly. Several in vitro methods were utilized to mimic protein self-organization in the cell. Our efforts resulted in reconstitution of the whole recombinant stalk in solution as well as on the ribosomal particle. On the basis of our analysis, it can be inferred that the P1A-P2B protein complex may be regarded as the key element in stalk formation, having structural and functional importance, whereas P1B-P2A protein complex is implicated in regulation of stalk function. The mechanism of quaternary structure formation could be described as a sequential co-folding/association reaction of an oligomeric system with P0-(P1A-P2B) protein complex as an essential element in the acquisition of a stable quaternary structure of the ribosomal stalk. On the other hand, the P1B-P2A complex is not involved in the cooperative stalk formation and our results indicate an increased rate of protein synthesis due to the latter protein pair.

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.

In vitro interaction of eukaryotic elongation factor 2 with synthetic oligoribonucleotide that mimics GTPase domain of rat 28S ribosomal RNA

Eukaryotic elongation factor 2 (eEF2) catalyzed the translocation of peptidyl-tRNA from the ribosomal A site to the P site. In this paper, the interaction between eEF2 and GTD RNA, a synthetic oligoribonucleotide that mimicked the GTPase domain of rat 28S ribosomal RNA, was studied in vitro. The purified eEF2 could bind to GTD RNA, forming a stable complex. Transfer RNA competed with GTD RNA in binding to eEF2, whereas poly(A), poly(U) and poly(I, C) did not interfere with the interaction between eEF2 and GTD RNA, demonstrating that the tertiary structure of RNA might be necessary for the recognition of and binding to eEF2. The complex formation of eEF2 with GTD RNA was inhibited by SRD RNA, a synthetic oligoribonucleotide mimic of Sarcin/Ricin domain RNA of rat 28S RNA. Similarly, GTD RNA inhibited the interaction between eEF2 and SRD RNA. This fact implies that these small oligoribonucleotides probably share similar recognition or binding identity elements in their tertiary structures. In addition, the binding of eEF2 to GTD RNA could be obviously weakened by the ADP-ribosylation of eEF2 with diphtheria toxin. These results indicate that eEF2 behaves differently from prokaryotic EF-G in binding to ribosomal RNA.

Pivotal role of the P1 N-terminal domain in the assembly of the mammalian ribosomal stalk and in the proteosynthetic activity

In the 60 S ribosomal subunit, the lateral stalk made of the P-proteins plays a major role in translation. It contains P0, an insoluble protein anchoring P1 and P2 to the ribosome. Here, rat recombinant P0 was overproduced in inclusion bodies and solubilized in complex with the other P-proteins. This method of solubilization appeared suitable to show protein complexes and revealed that P1, but not P2, interacted with P0. Furthermore, the use of truncated mutants of P1 and P2 indicated that residues 1-63 in P1 connected P0 to residues 1-65 in P2. Additional experiments resulted in the conclusion that P1 and P2 bound one another, either connected with P0 or free, as found in the cytoplasm. Accordingly, a model of association for the P-proteins in the stalk is proposed. Recombinant P0 in complex with phosphorylated P2 and either P1 or its (1-63) domain efficiently restored the proteosynthetic activity of 60 S subunits deprived of native P-proteins. Therefore, refolded P0 was functional and residues 1-63 only in P1 were essential. Furthermore, our results emphasize that the refolding principle used here is worth considering for solubilizing other insoluble proteins.

A specific role for the phosphorylation of mammalian acidic ribosomal protein P2

The acidic ribosomal proteins P1-P2 from rat liver were overproduced for the first time by expression of their cDNA in Escherichia coli. They were tested for their ability to reactivate inactive P1-P2-deficient core particles derived from 60 S ribosomal subunits treated with dimethylmaleic anhydride, in poly(U)-directed poly(Phe) synthesis. The recombinant P1-P2 were unable to reactivate these core particles although they could bind to them. When recombinant P1-P2 had been phosphorylated first with casein kinase II, they were as efficient in the reactivation process as P1-P2 extracted with ethanol/KCl from the 60 S subunits. Reconstitution experiments were carried out using all possible combinations of the two recombinant proteins phosphorylated or not. Reactivation of the core particles required the presence of both P1 and P2 with the latter in its phosphorylated form. These experiments reveal a distinct role for P1 and P2 in protein synthesis. Phosphorylated P2 produced a partial quenching of the intrinsic fluorescence of eukaryotic elongation factor 2, which was not observed with the unphosphorylated protein. This result demonstrates the existence of an interaction between phosphorylated P2 and eukaryotic elongation factor 2. P2 also quenched part of the intrinsic fluorescence of P1, due to the interaction between the two proteins.

Photoaffinity labeling of elongation factor-2 with 8-azido derivatives of GTP and ATP

Elongation factor 2 (eEF-2) can interact not only with guanylic nucleotides but also with adenylic ones, as was shown by intrinsic fluorescence quenching studies [Sontag, B., Reboud, A.M., Divita, G., Di Pietro, A., Guillot, D. & Reboud, J.P. (1993) Biochemistry 32, 1976-1980]. Here we studied sites of these interactions by using photoactivable 8-azido-[gamma-32P]GTP and 8-azido-[gamma-32P]ATP. Photoincorporation of the radioactive GTP derivative into eEF-2 was prevented by the previous addition of GTP and GDP. The addition of adenylic nucleotides (ATP, ADP) and some adenylic derivatives [NAD+, NADH,poly(A)] decreased the photoincorporation by only 40% at most. However, photoincorporation of the radioactive ATP derivative was prevented by the previous addition not only of adenylic compounds [ATP, ADP, NAD+, NADH, poly(A)] but also of GTP and GDP. Photoincorporation of radioactive nucleotide derivatives was not decreased by the addition of other nucleotidic compounds [UTP, poly(U), ITP, NADP+, NADPH]. ATP and GTP acted as non-competitive inhibitors of the photoincorporation of 8-azido-[gamma-32P]GTP and 8-azido-[gamma-32P]ATP, respectively. eEF-2 photolabeled with these radioactive nucleotide derivatives was submitted to trypsin digestion under different conditions and the labeled peptidic fragments identified after HPLC purification and gel electrophoresis by N-terminal sequencing. An octapeptide, Y264FDPANGK271, was the only peptide photolabeled with 8-azido-[gamma-32P]GTP whereas a N-terminal fragment of about 7 kDa was the only one photolabeled with 8-azido-[gamma-32P]ATP. The different results support the hypothesis that guanylic and adenylic nucleotides do not interact with the same site of eEF-2.

Properties of elongation factor-2 fragments obtained by partial proteolysis

Rat liver elongation factor eEF-2 was treated with endoproteinase Glu-C. Two major fragments were obtained, which were identified by N-terminal sequencing and purified. The larger one (F61) contained 554 residues including the N-terminal end, and after a second cleavage released a N-terminal peptide (F7) of 62 residues. The smaller one (F34) contained the other 303 residues including the C terminal end. F61 and F34, either isolated or after combination, were unable to catalyze protein synthesis. However, we show by fluorimetry that F61 could still interact with GTP and GDP. This fragment was was able to participate into a ternary complex with ribosome and GDP, but not with ribosome and a GTP analogue. It was unable to protect the ribosome against ricin-inactivation and to be phosphorylated by the eEF-2-specific Ca(2+)-calmodulin-dependent kinase, though it contained Trp221 and Thr56 involved in these reactions. On the other hand, F34 could be ADP-ribosylated in the presence of NAD+ and diphtheria toxin, but this fragment was apparently unable to bind to ribosomes. These results and those obtained with other proteinases are discussed in the light of the data published recently which show the existence of five different domains in the three-dimensional structure of EF-G.

Mode of action of bolesatine, a cytotoxic glycoprotein from Boletus satanas Lenz. Mechanistic approaches

Bolesatine is a potent cytotoxic glycoprotein purified from Boletus satanas Lenz, which has previously been shown to be an inhibitor of protein synthesis in several in vitro systems and in vivo. For a better understanding of its mechanism of action on protein synthesis at the ribosomal level, rat liver ribosomes were pretreated with bolesatine (1 to 10 micrograms) added to in vitro polyuridylic acid (poly(U)) translation systems before and after washing. The fact that ribosomes were still active confirmed that bolesatine cannot be included in the group of protein synthesis inhibitors of plant origin, known as ribosome-inactivating proteins (RIPs). The effect of bolesatine on the EF-2 elongation factor and post-ribosomal fraction was then studied in vitro. The results indicated that bolesatine does not have a direct effect on elongation factors, but hydrolyses the nucleoside triphosphates, GTP (80% to 90%, respectively for 1 to 10 micrograms) and ATP (10% to 40%, respectively for 1 to 10 micrograms), with consequent inhibition of protein synthesis. Thus, bolesatine should be classified as a nucleoside triphosphate phosphatase, rather than as a direct inhibitor of protein synthesis. The study of the effect of bolesatine on the EF-2 factor revealed that the mechanism whereby bolesatine affects protein synthesis probably involves GTP hydrolysis rather than EF-2 inhibition.

Peptide-chain elongation in eukaryotes

The elongation phase of translation leads to the decoding of the mRNA and the synthesis of the corresponding polypeptide chain. In most eukaryotes, two distinct protein elongation factors (eEF-1 and eEF-2) are required for elongation. Each is active as a complex with GTP. eEF-1 is a multimer and mediates the binding of the cognate aminoacyl-tRNA to the ribosome, while eEF-2, a monomer, catalyses the movement of the ribosome relative to the mRNA. Recent work showing that bacterial ribosomes possess three sites for tRNA binding and that during elongation tRNAs may occupy 'hybrid' sites is incorporated into a model of eukaryotic elongation. In fungi, elongation also requires a third factor, eEF-3. A number of mechanisms exist to promote the accuracy or 'fidelity' of elongation: eEF-3 may play a role here. cDNAs for this and the other elongation factors have been cloned and sequenced, and the structural and functional properties of the elongation factors are discussed. eEF-1 and eEF-2 can be regulated by phosphorylation, and this may serve to control rates of elongation in vivo.

Intrinsic tryptophan fluorescence of rat liver elongation factor eEF-2 to monitor the interaction with guanylic and adenylic nucleotides and related conformational changes

Elongation factor 2 (eEF-2), which contains seven Trp residues, exhibited a tryptophan-characteristic intrinsic fluorescence with maximum excitation at 280 nm and an emission peak centered at 333 nm that suggested a hydrophobic environment of these tryptophans. Upon denaturation with 6 M guanidine hydrochloride, the maximum emission was shifted to 348 nm. Fluorescence quenching studies using acrylamide and iodide confirmed that the Trp residues were mainly buried in the native molecule and indicated an important heterogeneity, the fractional accessible fluorescence (fa) values being 0.50 and 0.25, respectively. Partial quenching of eEF-2 fluorescence by nucleotides proved the existence of an interaction of the factor in the absence of ribosomes, not only with GDP but also with GTP, nonhydrolyzable analogs, GMP, and adenylic, but not cytidylic, nucleotides. Saturating binding plots showed different maximal changes of fluorescence depending upon the nucleotides, from 6.4% with ADP to 24.5% with GDP, and suggested the existence of more than one binding site for each nucleotide. Among all the nucleotides tested, only GTP at saturating concentration modified the fa value obtained with acrylamide (-36%). The possibility that this modification is related to a conformational change of eEF-2 induced by GTP binding is discussed.

Binding of GDP to a ribosomal protein after elongation factor-2 dependent GTP hydrolysis

Incubation of 80S ribosomes with a substoichiometric amount of [alpha-32P]GTP and with eEF-2 resulted in the specific labeling of one ribosomal protein which migrated very close to the position of the acidic phosphoprotein P2 from the 60S subunit in two-dimensional isofocusing-SDS gel electrophoresis. Localization of protein P2 in this electrophoretic system was ascertained by correlation with its position in the standard two-dimensional acidic-SDS gel electrophoresis after its specific phosphorylation by casein kinase II. Labeling of the ribosomal protein was dependent on the presence of eEF-2, and could be attributed to [alpha-32P]GDP binding from the results of chase experiments and HPLC identification, this binding being very likely responsible for the slight shift in the electrophoretical position of the protein. Incubation of ribosomes with tRNA(Phe) in the absence of mRNA induced the release of the bound GDP.

Effect of ADP-ribosylation and phosphorylation on the interaction of elongation factor 2 with guanylic nucleotides

Samples of unmodified EF-2, EF-2 ADP-ribosylated with diphtheria toxin and NAD, and/or phosphorylated using ATP and the Ca(2+)-calmodulin dependent kinase III partially purified, were irradiated at 254 nm with 32P-labeled GDP or GTP, and analyzed by one- and two-dimensional gel electrophoresis. By this method we showed that unmodified EF-2 formed a stable complex with GDP but not with GTP, whereas phosphorylated EF-2 and ADP-ribosylated + phosphorylated EF-2 formed stable complexes even in the absence of irradiation, with GTP but not GDP. ADP-ribosylated EF-2 did not form stable complexes with either GDP or GTP. Prior ADP-ribosylation of EF-2 increased its ability to the phosphorylated. These results show that the structures of the two domains containing diphtamide 715 and the phosphorylatable threonines (between Ala 51 and Arg 60) are interdependent; modifications of these residues induce different conformational changes of EF-2 which alter the interactions of the factor with guanylic nucleotides as well with ribosomes.

Modification of the accessibility of ribosomal proteins after elongation factor 2 binding to rat liver ribosomes and during translocation

Free- and EF-2-bound 80 S ribosomes, within the high-affinity complex with the non-hydrolysable GTP analog: guanylylmethylenediphosphonate (GuoPP(CH2)P), and the low-affinity complex with GDP, were treated with trypsin under conditions that modified neither their protein synthesis ability nor their sedimentation constant nor the bound EF-2 itself. Proteins extracted from trypsin-digested ribosomes were unambiguously identified using three different two-dimensional gel electrophoresis systems and 5 S RNA release was checked by submitting directly free- and EF-2-bound 80 S ribosomes, incubated with trypsin, to two-dimensional gel electrophoresis. Our results indicate that the binding of (EF-2)-GuoPP[CH2]P to 80 S ribosomes modified the behavior of a cluster of five proteins which were trypsin-resistant within free 80 S ribosomes and trypsin-sensitive within the high-affinity complex (proteins: L3, L10, L13a, L26, L27a). As for the binding of (EF-2)-GDP to 80 S ribosomes, it induced an intermediate conformational change of ribosomes, unshielding only protein L13a and L27a. Quantitative release of free intact 5 S RNA which occurred in the first case but not in the second one, should be related to the trypsinolysis of protein(s) L3 and/or L10 and/or L26. Results were discussed in relation to structural and functional data available on the ribosomal proteins we found to be modified by EF-2 binding.

Modification of the reactivity of three amino-acid residues in elongation factor 2 during its binding to ribosomes and translocation

The accessibility of three amino acids of EF-2, located within highly conserved regions near the N- and C-terminal extremities of the molecule (the E region and the ADPR region, respectively) to modifying enzymes has been compared within nucleotide-complexed EF-2 and ribosomal complexes that mimic the pre- and posttranslocational ones: the high-affinity complex (EF-2)-nonhydrolysable GTP analog GuoPP[CH2]P ribosome and the low-affinity (EF-2)-GDP-ribosome complex, EF-2 and ribosomes being from rat liver. We studied the reactivity of two highly conserved residues diphthamide-715 and Arg-66, to diphtheria-toxin-dependent ADP-ribosylation and trypsin attack, and of a threonine that probably lies between residues 51 and 60, to phosphorylation by a Ca2+/calmodulin-dependent protein kinase. Diphthamide 715 and this threonine residue were unreactive within the high-affinity complex but seemed fully reactive in the low-affinity complex. Arg-66 was resistant to trypsin in both complexes. The possible involvement of the E and ADPR regions of EF-2 in the interaction with ribosome in the two complexes is discussed.

Characterization of the ribosomal properties required for formation of a GTPase active complex with the eukaryotic elongation factor 2

The binding stability of the different nucleotide-dependent and -independent interactions between elongation factor 2 (EF-2) and 80S ribosomes, as well as 60S subunits, was studied and correlated to the kinetics of the EF-2- and ribosome-dependent hydrolysis of GTP. Empty reconstituted 80S ribosomes were found to contain two subpopulations of ribosomes, with approximately 80% capable of binding EF-2.GuoPP[CH2]P with high affinity (Kd less than 10(-9) M) and the rest only capable of binding the factor-nucleotide complex with low affinity (Kd = 3.7 x 10(-7) M). The activity of the EF-2- and 80S-ribosome dependent GTPase did not respond linearly to increasing factor concentrations. At low EF-2/ribosome ratios the number of GTP molecules hydrolyzed/factor molecule was considerably lower than at higher ratios. The low response coincided with the formation of the high-affinity complex. At increasing EF-2/ribosome ratios, the ribosomes capable of forming the high-affinity complex was saturated with EF-2, thus allowing formation of the low-affinity ribosome.EF-2 complex. Simultaneously, the GTPase activity/factor molecule increased, indicating that the low-affinity complex was responsible for activating the GTP hydrolysis. The large ribosomal subunits constituted a homogeneous population that interacted with EF-2 in a low-affinity (Kd = 1.3 x 10(-6) M) GTPase active complex, suggesting that the ribosomal domain responsible for activating the GTPase was located on the 60S subunit. Ricin treatment converted the 80S particles to the type of conformation only capable of interacting with EF-2 in a low-affinity complex. The structural alteration was accompanied by a dramatic increase in the EF-2-dependent GTPase activity. Surprisingly, ricin had no effect on the factor-catalyzed GTP hydrolysis in the presence of 60S subunits alone.

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.

Topography and stoichiometry of acidic phosphoproteins in rat liver 60 S ribosomal subunit

In reconstitution experiments of active 60 S subunits from inactive core particles obtained by using dimethyl maleic anhydride (DMMA), we observed that the phosphoproteins P1-P2 were extracted from the subunit by DMMA as a complex with other proteins. This complex was separated by electrophoresis and zonal centrifugation and shown, after 125I iodination of its components, to contain L22 and S12 in addition to P1-P2. Results suggest that it contains two copies of P1-P2 for one of L22 and S12.

Characterisation of the ribosomal binding site for eukaryotic elongation factor 2 by chemical cross-linking

Ribosomal complexes containing elongation factor 2 (EF-2) were formed by incubation of 80 S ribosomes in the presence of EF-2 and the non-hydrolysable GTP analogue GuoPP[CH2]P. The factor was covalently coupled to the ribosomal proteins located at the factor binding site, by treatment with bifunctional reagents. After isolation of the covalent EF-2.ribosomal protein complexes, the proteins were labelled with 125I and the introduced covalent links cleaved. The ribosomal proteins were identified by electrophoresis in two independent two-dimensional gel systems, followed by autoradiography. After cross-linking with bis(hydroxysuccinimidyl) tartrate (4 A between the reactive groups), protein S3/S3a, S7 and S11 were found as the major ribosomal proteins covalently linked to EF-2. The longer reagent, dimethyl 3,8-diaza-4,7-dioxo-5,6-dihydroxydecanbisimidate (11 A between the reactive groups), covalently coupled proteins S7, S11, L5, L13, L21, L23, L26, L27a and L32 to EF-2. After cross-linking with dimethyl suberimidate (9 A between the reactive groups) proteins S3/3a, S7, S11, L5, L8, L13, L21, L23, L26, L27a, L31 and L32 were identified as belonging to the EF-2-binding site. The results indicate that the ribosomal domain interacting with EF-2 is located on both the small and the large ribosomal subunit close to the subunit interface.

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.