Disulfide interchange reactions in 11S globulin subunits of Cruciferae seeds. Relationships to gene families.

Cruciferin, the main storage protein in rapeseed (Brassica napus L.), is a legumin-like 11S globulin. Using SDS/PAGE cruciferin was shown to be composed of different subunits consisting of alpha S and beta S polypeptides, which were disulfide linked, and also closely related free alpha f and beta f polypeptides which were not covalently linked. The origin of these latter free chains was found to be the result of disulfide interchange reactions occurring during SDS-induced dissociation of the hexameric 11S complex. These reactions involved the rupture of the alpha S-beta S interchain disulfide link and, as shown by specific thiol fluorescent labeling, the subsequent formation of an intra-beta-chain disulfide bond. N-terminal amino acid sequence determinations revealed that these reactions were related to particular cruciferin subunit beta chains having an additional cysteine residue (position 11) near the residue (position 7) implicated in the inter-chain disulfide bridges. Such a sequence could explain the origin of the free and linked polypeptides as also shown in SDS/PAGE analysis of radish (Raphanus sativum L.) and Arabidopsis thaliana H. cruciferins. These two polypeptide classes may be considered to represent phenotypic biochemical markers of two different Cruciferae globulin gene families.