ARABIDOPSIS-P40 HOMOLOG - A NOVEL ACIDIC PROTEIN ASSOCIATED WITH THE 40-S-SUBUNIT OF RIBOSOMES

We have isolated a full-length cDNA clone from Arabidopsis thaliana th at has extensive homology to p40 proteins from other organisms. The cD NA predicts a protein (A-p40) of 298 amino acids with a calculated mol ecular mass of 32.5 kDa and a pI of 4.8. Antibodies raised against an A-p40 fusion protein detected a polypeptide of about 40 kDa in Arabido psis and about 42 kDa in corn and soybean. In two-dimensional gel elec trophoresis the antibodies detected several acidic isoforms. A-p40 app ears to be located in the cytoplasm in two forms: soluble, or tightly associated with 40 S subunits and polysomes. Mixing experiments with s oluble A-p40 and purified corn polysomes showed that the ribosome-asso ciated form does not result from nonspecific binding during extraction . High concentrations of KCl (1.5 M) are needed to release A-p40 from ribosomes, which suggests that it may be a true ribosomal protein. Sev eral pieces of evidence suggest that A-p40 belongs to the acidic class of ribosomal proteins; first, its acidic pI; second, the presence of amino acid repeats conserved in other acidic ribosomal proteins; and t hird, the fact that it exists in the cell in two pools (free and ribos ome-associated).