ACIDIC PHOSPHOPROTEIN COMPLEX OF THE 60S RIBOSOMAL-SUBUNIT OF MAIZE SEEDLING ROOTS - COMPONENTS AND CHANGES IN RESPONSE TO FLOODING

We determined that ribosomes of seedling roots of maize (Zea mays L.) contain the acidic phosphoproteins (P-proteins) known to form a flexib le lateral stalk structure of the 60S subunit of eukaryotic ribosomes. The P-protein stalk, composed of PO, P1, and P2, interacts with elong ation factors, mRNA, and tRNA during translation. Acidic proteins of 1 3 to 15.5 kD were released as a complex from ribosomes with 0.4 M NH4C l/50% ethanol. Protein and cDNA sequence analysis confirmed that maize ribosomes contain one type of P1, two types of P2, and a fourth and n ovel P1/P2-type protein. This novel P-protein, designated P3, has the conserved C terminus of P1 and P2. P1, P2, and P3 are similar in deduc ed mass (11.4-12.2 kD) and isoelectric point (4.1-4.3). A 35.5-to 36-k D acidic protein was released at low levels from ribosomes with 1.0 M NH4Cl/50% ethanol and identified as P0. Labeling of roots with [P-32]i norganic phosphate confirmed the in vivo phosphorylation of the P-prot eins. Flooding caused dynamic changes in the P-protein complex, which affected the potential of ribosome-associated kinases and casein kinas e II to 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.