PROTEINS P1, P2, AND P0, COMPONENTS OF THE EUKARYOTIC RIBOSOME STALK - NEW STRUCTURAL AND FUNCTIONAL-ASPECTS

The eukaryoic ribosomal stalk is thought to consist of the phosphoprot eins P1 and P2, which form a complex with protein P0. This complex int eracts at the GTPase domain in the large subunit rRNA, overlapping the binding site of the protein L11-like eukaryotic counterpart (Saccharo myces cerevisiae protein L15 and mammalian protein L12). An unusual po ol of the dephosphorylated forms of proteins P1 and P2 is detected in eukaryotic cytoplasm, and an exchange between the proteins in the pool and on the ribosome takes place during translation. Quadruply disrupt ed yeast strains, carrying four inactive acidic protein genes and, the refore, containing ribosomes totally depleted of acidic proteins, are viable but grow with a doubling time threefold higher than wild-type c ells. The in vitro translation systems derived from these stains are a ctive but the two-dimensional gel electrophoresis pattern of proteins expressed in vivo and in vitro is partially different. These results i ndicate that the P1 and P2 proteins are not essential for ribosome act ivity but are able to affect the translation of some specific mRNAs. P rotein P0 is analogous to bacterial ribosomal protein L10 but carries an additional carboxyl domain showing a high sequence homology to the acidic proteins P1 and P2, including the terminal peptide DDDMGFGLFD. Successive deletions of the P0 carboxyl domain show that removal of th e last 21 amino acids from the P0 carboxyl domain only slightly affect s the ribosome activity in a wild-type genetic background; however, th e same deletion is lethal in a quadruple disruptant deprived of acidic P1/P2 proteins. Additional deletions affect the interaction of P0 wit h the P1 and P2 proteins and with the rRNA. The experimental data avai lable support the implication of the eukaryotic stalk components in so me regulatory process that modulates the ribosomal activity.