Characterization of the 12S globulin complex of Brassica napus. Evolutionary relationship to other 11-12S storage globulins.

Cruciferin (12S globulin) is the major seed protein in Brassica napus (oil seed rape). It is synthesized during seed development and consists of six subunit pairs. Each of these pairs is synthesized as a precursor containing one alpha and one beta chain. At least three different precursors exist (P1-3), giving rise to four different mature subunits (cru1-4). Several cruciferin clones were isolated from a seed mRNA cDNA library. Comparison of the deduced amino acid sequences of these clones to amino acid sequences of purified cruciferin chains and peptides identified them as coding for cru2/3 and cru4 subunits. From the amino acid sequences deduced from two overlapping cDNA clones, the precursor of the cru4 subunit was shown to consist of 465 amino acid residues. Comparison of cruciferin and cruciferin-related sequences from B. napus and Arabidopsis thaliana, respectively, suggested that early during evolution the Brassicaceae family only possessed two types of 11-12S globulin genes, like the present-day Fabaceae.