STORAGE GLOBULINS SYNTHESIZED INVITRO BY POLYRIBOSOMES AND MESSENGER-RNA FROM DEVELOPING SEEDS OF LUPINUS-LUTEUS

The synthesis and modification of the storage proteins of Lupinus luteus have been investigated by means of the wheat germ-derived cell-free system programmed either by poly(A)+ RNA or polyribosomes attached to membranes of endoplasmic reticulum. Polypeptides related to conglutins alpha, beta and gamma were identified by immunoprecipitation, followed by sodium dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and fluorography. The legumin-like conglutin-alpha, synthesised by poly (A)+ RNA is represented by a family of precursor polypeptides of Mr 48,000-78,000, whereas the vicilin-like conglutin-beta as a group of precursors of Mr 44,000-76,000. Primary precursor polypeptide of lupin specific conglutin-gamma has a Mr 51,000. During lupin embryogenesis a differential expression of genes encoding storage globulins was observed, e. g. the mRNA for vicilin-like conglutin-beta appeared earlier in maturing seeds than the message coding for conglutin-alpha. The comparison of products synthesised by poly (A)+ RNA in the absence and presence of exogenous microsomal membranes and by membrane-bound polyribosomes shows that all conglutins, like secretory proteins, are synthesised as pre-proteins with a signal peptide, which had been co-translationally cleaved. The precursor polypeptides of three investigated storage proteins synthesised in vitro differ from subunits of conglutin-alpha, beta and gamma found in dry seeds. The models showing the relationships between subunits of conglutin-alpha, beta and gamma and their precursors are proposed.