Arabidopsis p40 homologue. A novel acidic protein associated with the 40 S subunit of ribosomes.

We have isolated a full-length cDNA clone from Arabidopsis thaliana that has extensive homology to p40 proteins from other organisms. The cDNA predicts a protein (A-p40) of 298 amino acids with a calculated molecular 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 Arabidopsis and about 42 kDa in corn and soybean. In two-dimensional gel electrophoresis the antibodies detected several acidic isoforms. A-p40 appears to be located in the cytoplasm in two forms: soluble, or tightly associated with 40 S subunits and polysomes. Mixing experiments with soluble A-p40 and purified corn polysomes showed that the ribosome-associated 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. Several 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 third, the fact that it exists in the cell in two pools (free and ribosome-associated).