Phosphorylation of ribosomal protein P0 is not essential for ribosome function but can affect translation

Protein P0, an essential component of the eukaryotic ribosomal stalk, is fo und phosphorylated in the ribosome. Substitution of serine 302 in the amino acid sequence of the Saccharomyces cerevisiae P0 by either aspartic acid o r cysteine abolishes in vitro and in vivo phosphorylation of the protein. O n the contrary, the replacement of this serine by a threonine results in an increase in the protein phosphorylation under both sets of conditions. The refore, this serine residue, which is part of a consensus casein kinase II modification site, SDDD, seems to be the phosphorylation site in protein P0 . The effect of the mutations on the protein activity has been tested in S. cerevisiae W303dGP0 and D67dGP0, both of which carry a genomic P0 gene und er the control of the GAL1 promoter. Transformation of the mutated genes in S. cerevisiae W303dGP0 allows cell growth at 30 degrees C in glucose-to re press the wild-type P0 expression-at the same rate as controls, and the rib osomes contain a normal amount of the other stalk components. A similar abs ence of effect of the mutations on growth was found in strain D67dGP0, whic h has ribosomes deprived of the P1 and P2 proteins. Therefore, P0 phosphory lation is not a requirement for ribosome activity in standard growth condit ions either in the presence or in the absence of the other stalk proteins. However, a phenotypic effect is detected in the case of strain D67 transfor med with the overphosphorylated threonine containing P0, which contrary to the wild-type and the other mutated proteins is unable to support cell grow th at 37 degrees C in the presence of either 0.3 M NaCl or 0.8 M sorbitol. In vitro polymerizing tests indicate that this effect is not due to the the rmosensitivity of the mutated protein. The results indicate that although P 0 phosphorylation is not required for the overall ribosome activity, it may affect the expression of specific proteins involved in metabolic processes such as osmoregulation.