We determined that ribosomes of seedling roots of maize (Zea mays 1.) contain the acidic phosphoproteins (P-proteins) known to form a flexible lateral stalk structure of the 60s subunit of eukaryotic ribosomes. l h e P-protein stalk, composed of PO, P1, and P2, interacts with elongation factors, mRNA, and tRNA during translation. Acidic proteins of 13 to 15.5 k D were released as a complex from ribosomes with 0.4 M NH,CI/SO% ethanol. Protein and cDNA sequence analysis confirmed that maize ribosomes contain one type of P1, two types of P2, and a fourth and novel Pl/PZ-type protein.This novel P-protein, designated P3, has the conserved C terminus of P1 and P2. P1, P2, and P3 are similar in deduced m a s (1 1.4-1 2.2 kD) and isoelectric point (4.1-4.3). A 35.5-t o 36-kD acidic protein was released at low levels from ribosomes with 1 .O M NH,C1/50% ethanol and identified as PO. Labeling of roots with [32P]inorganic phosphate confirmed the in vivo phosphorylation of the P-proteins.Flooding caused dynamic changes in the P-protein complex, which affected the potential of ribosome-associated kinases and casein kinase II t o phosphorylate the P-proteins. We discuss possible alterations of the ribosomal P-protein complex and consider that these changes may be involved in the selective translation of mRNA in flooded roots.Ribosomes are a two-subunit organelle, and are the site of mRNA translation into protein in a11 organisms. The large ribosomal subunit is a complex macromolecule that is composed of rRNAs, a large number of basic (high-pI) proteins, and a small number of acidic (low-pI) proteins. Across evolutionary kingdoms and phyla the large ribosoma1 subunit is variable in size, but possesses a number of morphological features that are universally conserved. For example, a universal feature of the peptidyl transferase region of the large subunit is a complex of acidic proteins that form the body and stalk of a lateral protuberance (Moller, 1990; Liljas, 1991).In bacteria the acidic protein stalk of the 50s ribosomal subunit is composed of ribosomal protein, L10, and two dimers of L7 and L12 in a (L7/L12),-LlO pentameric complex (Moller, 1990; Liljas, 1991). L10 is a 17-kD acidic pro- tein that interacts with the 23s rRNA scaffold of the large subunit within the GTPase domain of the rRNA. L7 and L12 are 12-kD acidic proteins that are encoded by a single gene, but differ in that the N-terminal Ser of L7 is posttranslationally aminoacetylated. The N-terminal domain of L7 and L12 forms an a-helical structure that is responsible for dimer formation and binding to M O . A central region of acidic residues forms a flexible hinge that allows the dimers to assume an elongated conformation that forms the stalk or a closed conformation in which the C-and N-terminal domains are in close proximity on the body of the 50s ribosomal subunit (Oleinikov et al., 1993;Traut et al., 1993). The C-terminal regions of L7 and L12 are required for binding of elongation factor G, and the subsequent hydrolysis of GTP that occurs in the transloca...

The P-protein complex of eukaryotic ribosomes forms a lateral stalk structure in the active site of the large ribosomal subunit and is thought to assist in the elongation phase of translation by stimulating GTPase activity of elongation factor-2 and removal of deacylated tRNA. The complex in animals, fungi, and protozoans is composed of the acidic phosphoproteins P0 (35 kDa), P1 (11-12 kDa), and P2 (11-12 kDa). Previously we demonstrated by protein purification and microsequencing that ribosomes of maize (Zea mays L.) contain P0, one type of P1, two types of P2, and a distinct P1/P2 type protein designated P3. Here we implemented distance matrices, maximum parsimony, and neighbor-joining analyses to assess the evolutionary relationships between the 12 kDa P-proteins of maize and representative eukaryotic species. The analyses identify P3, found to date only in mono-and dicotyledonous plants, as an evolutionarily distinct P-protein. Plants possess three distinct groups of 12 kDa P-proteins (P1, P2, and P3), whereas animals, fungi, and protozoans possess only two distinct groups (P1 and P2). These findings demonstrate that the P-protein complex has evolved into a highly divergent complex with respect to protein composition despite its critical position within the active site of the ribosome.Translation is a complex, multi-step process that involves ribosomes; initiation, elongation, and release factors; aminoacyl-tRNAs; mRNA; and mRNA-binding proteins (1-3). Ribosomes, which catalyze polypeptide synthesis, consist of 3-4 rRNA molecules and up to 90 proteins assembled into large and small subunits. Both prokaryotic and eukaryotic ribosomes have been investigated, with the eukaryotic emphasis on ribosomes of rat and yeast (4,5), and little detailed analysis of the ribosomes of plants (reviewed in ref. 6). The overall structure and function of the eukaryotic ribosome is considered to be conserved. The small ribosomal subunits of animals, fungi, and plants are very similar in molecular mass, whereas the mass of the large ribosomal subunits are quite variable. Large ribosomal subunits of plants have a lower molecular mass than that of rat or yeast (6, 7), which is due in part to nucleotide sequence differences in the 23S-like rRNA component (6, 7), but may also result from heterogeneity in ribosomal protein (r-protein) composition.The majority of r-proteins are basic (pI Ͼ 8.5). There are, however, a group of acidic r-proteins with isoelectric points in the pH 3-5 range, a subset of which form a distinct and universally conserved lateral-stalk structure on the large ribosomal subunit (8, 9). The stalk structure is present in the active site of the ribosome where interactions between mRNA, tRNA, and translation factors occur during the late initiation, elongation, and termination phases of translation (8). In Escherichia coli, the lateral stalk is a pentameric complex that contains one molecule of r-protein L10 (17 kDa) and two heterodimers of r-proteins L7 (12 kDa) and L12 (12 kDa) (10). The L7/L12 dimers attac...