An acidic protein associated to ribosomes of Saccharomyces cerevisiae changes during cell cycle

Identification of A1 protein as the fourth member of 13 kDa-type acidic ribosomal protein family in yeast Saccharomyces cerevisiae

The identity of protein A1 predicted by a cDNA clone from yeast Saccharomyces cerevisiae which has common carboxyl-terminus to 13 kDa-type acidic ribosomal proteins has been examined. The unique gene for A1 was isolated using the cDNA clone and found to possess two boxes similar to upstream activation sequences for ribosomal protein genes (UASrpg) in the 5'-flanking region. The in vitro-translation product directed by hybrid-selected mRNA with A1 cDNA comigrated with a minor component of split proteins from ribosome by electrofocusing. In addition, the mRNA level for A1 was found to be lower than other two major acidic ribosomal proteins suggesting that A1 is the fourth member of the protein family so far identified which is expressed at relatively low level.

Ribosomal protein interactions in yeast. Protein L15 forms a complex with the acidic proteins

Protein L15 from Saccharomyces cerevisiae ribosomes has been shown to interact in solution with acidic ribosomal proteins L44, L44' and L45 by different methods. Thus, the presence of the acidic proteins changes the elution characteristics of protein L15 from CM-cellulose and DEAE-cellulose columns and from reverse-phase HPLC columns. Moreover, immunoprecipitation using anti-L15 specific monoclonal antibodies coprecipitates the acidic proteins, too. Conversely, antibodies raised against the acidic proteins immunoprecipitate protein L15. This coprecipitation seems to be specific since it does not involve other ribosomal proteins present in the sample. Similarly, plastic-adsorbed antibodies specific for one of the components in the L15--acidic-protein complex are able to retain the other component of the complex but cannot bind unrelated proteins. Moreover, protein L15 can be chemically cross-linked to the acidic proteins in solution. These results indicate that protein L15 might be equivalent to bacterial ribosomal protein L10 in forming a complex with the acidic proteins. Since, on the other hand, protein L15 has been shown to be immunologically related to bacterial protein L11 [Juan Vidales et al. (1983) Eur. J. Biochem. 136, 276-281] and to interact with the same region of the large ribosomal RNA as does protein L11 [El-Baradi et al. (1987) J. Mol. Biol. 195, 909-917], these results suggest strongly that protein L15 plays the same role in the yeast ribosome as proteins L10 and L11 do in the bacterial particles.

Effects of different amino-group reagents on ribosomal integrity: structural role of lysine residues

Treatment of 60S subunits from yeast ribosomes with dicarboxylic acid anhydrides (maleic, dimethylmaleic and tetrahydrophtalic), which introduces negatively-charged residues, is accompanied by substantial dissociation of protein components (35-55%). In contrast, acetic anhydride or cyanate, which introduce uncharged groups, cause practically no protein release, even after extensive modification. Therefore, in addition to blocking lysine-RNA interactions, a large change in the electric charge of the proteins appears to be necessary to obtain dissociation. These results seem to indicate that lysine residues are not essential to ribosome integrity, while arginine-RNA interactions should play an important role in the maintenance of ribosomal structure.

Peptidyl transferase centres of rat and yeast ribosomes. Different response to modification of protein amino groups

Modification of rat liver ribosomes with dimethylmaleic anhydride, a reagent for protein amino groups, causes a large stimulation of peptidyl transferase activity assayed by the "fragment" reaction, as well as the inactivation of poly(U)-directed polyphenylalanine synthesis. In contrast to rat ribosomes, the peptidyl transferase of yeast ribosomes is little affected by modification. Although other interpretations are not excluded, these results might be due to differences between the peptidyl transferase centres of mammalian and yeast ribosomes.

Partial reconstitution of 60S ribosomal subunits from yeast

Ribosomal 60S subunits active in polyphenylalanine synthesis can be reconstituted from core particles lacking 20-40% of the total protein. These core particles were obtained by treatment of yeast 60S subunits with dimethylmaleic anhydride, a reagent for protein amino groups. Upon reconstitution a complementary amount of split proteins is incorporated into the ribosomal particles, which have the sedimentation coefficient of the original subunits. Ribosomal protein fractions obtained by extraction with 1.25 M NH4Cl, 4 M LiCl, 7 M LiCl, or 67% acetic acid, are much less efficient in the reconstitution of active subunits from these core particles than the corresponding released fraction prepared with dimethylmaleic anhydride. Attempts to reconstitute active subunits from protein-deficient particles obtained with 1.25 M NH4Cl plus different preparations of ribosomal proteins, including the fraction released with dimethylmaleic anhydride, were unsuccessful. Therefore, under our conditions, of the disassembly procedures assayed only dimethylmaleic anhydride allows partial reconstitution of active 60S subunits.

A cytoplasmic, cyclic nucleotide-independent casein kinase II from Saccharomyces cerevisiae

Two molecular forms of casein kinase II (an ATP: protein phosphotransferase, EC 2.7.1.37) from yeast were isolated and characterized. The first form was composed of three polypeptide subunits with molecular weights of 41000, 37000 and 24000. The second form contained two larger polypeptides and lacked an autophosphorylatable 24 kDa subunit. The properties of both enzyme forms were found to be practically the same in respect to the substrate and phosphate donor specificities, kinetics, their sensitivity to heparin, etc. The results obtained strongly indicate that isolated yeast casein kinase II does not necessarily require the smallest subunit for the enzyme activity.

Effect of phosphorylation on the affinity of acidic proteins from Saccharomyces cerevisiae for the ribosomes

Electrofocusing of the acidic proteins extracted from Saccharomyces cerevisiae ribosomes shows the presence of eight bands in the gels, which upon treatment with alkaline phosphatase are reduced to three. Two of them, proteins L44 and L45, correspond to the proteins equivalent to the bacterial L7 and L12 and the third, protein Ax, behaves like a supernatant factor. In the ribosome, proteins L44 and L45 are found unphosphorylated and monophosphorylated while protein Ax is detected mostly in a modified state, showing from one to three phosphate groups per molecule. In the cytoplasm where protein Ax is abundant and proteins L44 and L45 are present in small quantities, the three proteins are unphosphorylated. Protein Ax, having one or two phosphate groups, can be removed from the ribosomes in conditions that release the initiation factors, while the triphosphorylated molecules are tightly bound to the particles. The data indicate a relationship between the degree of phosphorylation of protein Az and its affinity for the ribosome.