IDENTIFICATION OF NONRIBOSOMAL POLYPEPTIDES IN THE POLYSOME PREPARATIONS FROM VICIA-FABA L LEAVES

The compositions of proteins in the preparations of membrane-bound and free polysomes isolated from Vicia faba L. leaves of different ages w ere studied. The plants were grown under 10 or 125 W/m(2) PAR. Both fr ee and membrane-bound polysomes were shown to contain significant amou nts of two polypeptides with mol wts of 55 and 42 kD. The 55 kD polype ptide was found mainly in the free polysome fraction, whereas the 42 k D polypeptide was found in the membrane-bound polysomes. Distribution of these polypeptides between various subcellular fractions and their partitioning between polysomes, ribosomes, and ribosomal subunits frac tionated in the sucrose density gradients were monitored. The 55 kD po lypeptide was not associated with ribosomes but was involved in supram olecular structures not exceeding 40S, which were present in the ribos ome preparations. This was also a major polypeptide of the postribosom al supernatant. Enzyme immunoassays allowed the authors to identify 55 kD polypeptide as a large subunit of ribulose bisphosphate carboxylas e. The 42 kD polypeptide was absent from the postribosomal supernatant but closely followed the sedimentation pattern of ribosomes in the su crose gradient. Its relative content per ribosome, however, increased both with leaf age and at higher irradiance. This regularity was equal ly characteristic of both free and membrane-bound polysomes. Using imm unological techniques, we identified this polypeptide as actin. The as sociation between plant ribosomes and the actin cytoskeleton is assume d.