Ribosomal Acidic Phosphoproteins P1 and P2 Are Not Required for Cell Viability but Regulate the Pattern of Protein Expression in Saccharomyces cerevisiae

Abstract

Saccharomyces cerevisiae strains with either three inactivated genes (triple disruptants) or four inactivated genes (quadruple disruptants) encoding the four acidic ribosomal phosphoproteins, YP1␣, YP1␤, YP2␣, and YP2␤, present in this species have been obtained. Ribosomes from the triple disruptants and, obviously, those from the quadruple strain do not have bound P proteins. All disrupted strains are viable; however, they show a cold-sensitive phenotype, growing very poorly at 23؇C. Cell extracts from the quadruple-disruptant strain are about 30% as active as the control in protein synthesis assays and are stimulated by the addition of free acidic P proteins. Strains lacking acidic proteins do not have a higher suppressor activity than the parental strains, and cell extracts derived from the quadruple disruptant do not show a higher degree of misreading, indicating that the absence of acidic proteins does not affect the accuracy of the ribosomes. However, the patterns of protein expressed in the cells as well as in the cell-free protein system are affected by the absence of P proteins from the particles; a wild-type pattern is restored upon addition of exogenous P proteins to the cell extract. In addition, strains carrying P-protein-deficient ribosomes are unable to sporulate but recover this capacity upon transformation with one of the missing genes. These results indicate that acidic proteins are not an absolute requirement for protein synthesis but regulate the activity of the 60S subunit, affecting the translation of certain mRNAs differently.The presence of a set of strongly acidic proteins is a universal feature of ribosomes from all organisms. These proteins have been involved in interactions with different supernatant factors during protein synthesis in prokaryotic as well as eukaryotic systems (see reference 16 for a review).In eukaryotic organisms, the acidic ribosomal proteins, called generically P proteins, are capable of being phosphorylated, and this modification drastically affects the interaction of the polypeptides with the ribosome in vitro (11,14). In fact, this process might be part of a control mechanism that, by regulating the amount of bound protein, can affect the activity of the particles in relation to the metabolic requirements of the cell (24).In Saccharomyces cerevisiae, four acidic ribosomal proteins have been reported and characterized; according to structural and functional considerations, they can be classified in two groups, YP1␣/␤ and YP2␣/␤, corresponding to the two types of acidic polypeptides present in mammalian ribosomes (35). Using gene disruption techniques, we previously found that the individual elimination of each polypeptide, and even the double disruption of any two of the four proteins, is not a lethal event for the cell, although the generation time is affected to different extents (20, 21). The effect is especially important when the genes encoding the two members of the same group are disrupted, since in this case the remaining acidic proteins present in the c...

The ribosome filter hypothesis

“…In yeast, ribosomes from stationary phase are deficient in P-proteins when compared with those from exponentially growing cells (63). By using cell-free lysates, it was demonstrated that yeast ribosomes lacking P-proteins translated particular mRNAs differentially as compared with ribosomes containing P-proteins (64). The addition of exogenous P-proteins to lysates lacking them abrogated the specific translational differences, suggesting that differences in translation efficiency are attributable to heterogeneity in the P-proteins.…”

High heterogeneity within the ribosomal proteins of the Arabidopsis thaliana 80S ribosome

“…In comparison, the general tendency of the ribosomal proteins is very different, since the various spots belonging to a distinct gene, are generally distributed evenly in the horizontal direction (Figure 2), indicating a change only in their pI and not in their MW. An exception is the slightly different pattern observed for the ribosomal proteins P1, P2 and P3, which are collectively referred to as stalk proteins because they form a stalk-like protrusion on the large subunit (reviewed by Ballesta and Remacha, 1996) and have been reported to be modified in yeast (Remacha et al, 1995). This is seen particularly for the spot identified as P2 (compare for example the spots for the P2 protein (caption in Figure 1B) with the ones for the S27, L11 or L5 proteins (Figure 2)).…”

Increased Expression of Hsp40 Chaperones, Transcriptional Factors, and Ribosomal Protein Rpp0 Can Cure Yeast Prions

“…We suggest that the excess of Rpp0 may affect transcription by participating in it either directly or indirectly. The latter opportunity is related to the observation that the lack of acidic ribosomal proteins Rpp1A/B and Rpp2A/B, structurally and functionally related to Rpp0, significantly affected the expression of many proteins and chaperones in particular (46). It was proposed that the altered content of acidic proteins in ribosome changed its specificity toward different mRNAs.…”