Ribosomal protein P0, contrary to phosphoproteins P1 and P2, is required for ribosome activity and Saccharomyces cerevisiae viability

Abstract

Protein P0 in Saccharomyces cerevisiae is found only in the ribosomes and not free in a cytoplasmic pool like the structurally related acidic P1 and P2 proteins. Analogously, P0 stays bound to the particles in conditions that release the other P proteins. Attempts to obtain yeast strains carrying an interrupted P0 gene by direct gene disruption techniques of different yeast strains always resulted in haploid cells carrying one disrupted and one intact copy of the gene. Disruption of the unique P0 genomic copy seems to induce a duplication and occasionally a chromosomal transposition of the gene. Conditional null mutants of P0 were then constructed carrying the P0 gene under the control of the inducible GAL1 promoter. A 2-3-fold excess of P0 mRNA is found in the conditional mutant when grown in galactose; however, only a small increase of the P0 protein is detected in total cell extracts. No P0 protein is detected in the cell supernatant, indicating that, like the standard ribosomal proteins and opposite to the other P proteins, the protein not bound to the ribosomes is degraded. Transfer of the mutants to the restrictive conditions causes, after some generations, a growth stop that finally leads to cell death. The growth decline is paralleled by a reduction in the polysome number and the appearance of half-mer particles as well as by an accumulation of 60 S particles deficient in P0 and in the acidic proteins P1 and P2. These results indicate that P0 is required for the interaction of the acidic P1 and P2 proteins with the ribosomes, and in its absence, deficient 60 S ribosomes are assembled which are inactive in protein synthesis resulting in cell lethality.