Phosphorylation of the acidic ribosomal P proteins in Saccharomyces cerevisiae: A reappraisal

Previous reports had pointed to serines 62 and 71/79 as possible phosphorylation sites in the yeast acidic ribosomal proteins YP1 alpha and YP2 alpha, respectively. However, it has been found that mutation of these serine residues did not affect the phosphorylation level of the proteins. A detailed examination of the YP2 alpha tryptic digest from the in vivo labeled protein demonstrates the existence of a totally trypsin-insensitive site at lysine 88 that led to a misinterpretation of previous results. The unique YP2 alpha tryptic phosphopeptide obtained contains, in addition to serines 71 and 79, a serine at position 96 near the carboxyl end, which automatic Edman degradation confirmed as the phosphorylated residue. In addition, by using Staphyloccocus protease V8, it was possible to obtain phosphopeptides containing only serine 96, whose phosphorylation has likewise been confirmed by radioactive labeling as well as by chemical methods. A similar analysis of the other 12 kDa acidic proteins, YP1 alpha, YP1 beta, and YP2 beta, has shown the presence of equivalent phosphorylation sites in the four P proteins, which correspond to position 96 in proteins YP1 alpha, YP1 beta, and YP2 alpha and position 100 in YP2 beta. This conclusion has been confirmed by the fact that mutation of serine 96 in proteins YP1 alpha and YP2 alpha abolishes their capacity to be phosphorylated in vivo. The mutation of the phosphorylation site of the individual acidic proteins seems not to alter their interaction with the ribosome. However, it has been found that the level of phosphorylation of the stalk proteins has an effect on the response of the cells to some specific metabolic conditions, indicating that it may modulate the translation of specific proteins.