The legumin precursor from white lupin seed. Identity of the subunits, assembly and proteolysis

The precursors of the legumin-like storage protein from developing white lupin seeds (35 days after flowering) are trimers composed of protomers of M(r) 72,000 or 67,000. Some subunits of these oligomers contain processed precursor polypeptides, namely alpha polypeptides of either 52,000 or 44,000 linked through disulphide bonds to a beta polypeptide of 21,000, typical of the mature legumin. The prolegumin is glycosylated. Legumin oligomers purified from the same seeds are both trimers and hexamers; some of their subunits are still made of precursor polypeptides. The hexamer contains less precursor polypeptide than the trimer. A low level or absence of precursor appears to be a condition of hexamer assembly. The heterogenous prolegumin and legumin oligomers represent intermediates in the processing of the prolegumin to mature legumin. Hydrophobic-interaction chromatography on TSK-phenyl-5PW and titration with the hydrophobic probe 8-anilino-1-naphthalenesulphonate indicate that the legumin is less hydrophobic than the prolegumin. This is attributed to structural rearrangements at processing of the propolypeptide, made evident by the behaviour in CD and by the second-derivative ultraviolet spectra of the two proteins. The total protein extract of developing cotyledons at 40 days after flowering contains endopeptidases, similar to those existing in the resting seeds, which cause a limited cascade degradation of the prolegumin and legumin.