PHOSPHORYLATION OF THE ACIDIC RIBOSOMAL P-PROTEINS IN SACCHAROMYCES-CEREVISIAE - A REAPPRAISAL

Previous reports had pointed to serines 62 and 71/79 as possible phosp horylation sites in the yeast acidic ribosomal proteins YP1 alpha and YP2 alpha, respectively. However, it has been found that mutation of t hese serine residues did not affect the phosphorylation level of the p roteins. A detailed examination of the YP2 alpha tryptic digest from t he in vivo labeled protein demonstrates the existence of a totally try psin-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 n ear the carboxyl end, which automatic Edman degradation confirmed as t he phosphorylated residue. In addition, by using Staphyloccocus protea se V8, it was possible to obtain phosphopeptides containing only serin e 96, whose phosphorylation has likewise been confirmed by radioactive labeling as well as by chemical methods. A similar analysis of the ot her 12 kDa acidic proteins, YP1 alpha, YP1 beta, and YP2 beta, has sho wn the presence of equivalent phosphorylation sites in the four P prot eins, 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 prot eins seems not to alter their interaction with the ribosome. However, it has been found that the level of phosphorylation of the stalk prote ins has an effect on the response of the cells to some specific metabo lic conditions, indicating that it may modulate the translation of spe cific proteins.