Proteins P1, P2, and P0, components of the eukaryotic ribosome stalk. New structural and functional aspects

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Abstract
The eukaryoic ribosomal stalk is thought to consist of the phosphoproteins P1 and P2, which form a complex with protein P0. This complex interacts at the GTPase domain in the large subunit rRNA, overlapping the binding site of the protein L11-like eukaryotic counterpart (Saccharomyces cerevisiae protein L15 and mammalian protein LI2). An unusual pool 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 disrupted yeast strains, carrying four inactive acidic protein genes and, therefore, containing ribosomes totally depleted of acidic proteins, are viable but grow with a doubling time threefold higher than wild-type cells. The in vitro translation systems derived from these stains are active but the two-dimensional gel electrophoresis pattern of proteins expressed in vivo and in vitro is partially different. These results indicate that the P1 and P2 proteins are not essential for ribosome activity but are able to affect the translation of some specific mRNAs. Protein 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 the last 21 amino acids from the P0 carboxyl domain only slightly affects the ribosome activity in a wild-type genetic background; however, the same deletion is lethal in a quadruple disruptant deprived of acidic P1/P2 proteins. Additional deletions affect the interaction of P0 with the P1 and P2 proteins and with the rRNA. The experimental data available support the implication of the eukaryotic stalk components in some regulatory process that modulates the ribosomal activity.Key words: ribosomal stalk, acidic proteins, phosphorylation, GTPase domain, translation regulation.