THE HIGHLY CONSERVED PROTEIN P0 CARBOXYL END IS ESSENTIAL FOR RIBOSOME ACTIVITY ONLY IN THE ABSENCE OF PROTEINS P1 AND P2

Protein P0 together with proteins P1 and P2 form the stalk in eukaryot ic ribosomes. P0 has a carboxyl-terminal domain about 100 amino acids long that has high sequence similar to the ribosomal proteins P1 and P 2. By sequential deletion of this region, a series of Saccharomyces ce revisiae truncated P0 genes have been constructed that encode proteins lacking 21, 87, and 132 amino acids from the carboxyl terminus, respe ctively. These constructions have been used to transform yeast P0 cond itional null mutants to test their capacity to restore cell growth. Re moval of only the last 21 amino acids causes a small effect on cell gr owth in wild-type strains; however, this deletion is lethal in strains having P protein-deficient ribosomes. A P0 lacking 87 amino acids all ows cell growth at a low rate, and ribosomes bind P proteins with much less affinity. Lastly, removal of 132 amino acids totally inactivates P0; this deleted protein is unable to bind to the particles, causing a deficiency in active 60 S subunits and making the cell nonviable. Th ese results indicate that at least one out of the five protein P-like carboxyl termini present in the ribosome has to be firmly bound to the particle for protein synthesis and cell viability, and this structure can be provided by protein P0. The part of P0 from around positions 2 30-290 is important for the interaction of proteins P1/P2 with the rib osome, but it is not essential for protein synthesis. Finally, the reg ion including from residues 185 to 230 is required for the interaction of P0 with the rRNA.