Ric c 1 and Ric c 3, the allergenic 2S albumin storage proteins of Ricinus communis: complete primary structures and phylogenetic relationships.

The 2S albumin storage protein of Ricinus communis consists of the two heterodimeric proteins Ric c 1 and Ric c 3 each of which is composed of a small and a large subunit linked together by disulphide bridges. The complete primary structures of both heterodimeric proteins were determined by enzymatic degradation and automated Edman degradation. The sequences of all four chains correspond to the known cDNA sequence of the gene of a presumed precursor molecule and to the previously determined partial sequences for Ric c 1 and Ric c 3. In addition, few differences in amino acid positions were found which seem to be related to different varieties of R. communis. Sequence comparisons with 2S albumin from other plant genera revealed high degrees of homology and support the view of a common genetic origin of this protein family. Ric c 1 and Ric c 3 which have 11,212 and 12,032 daltons, respectively, share a similar molecular size, biological function and allergenicity with the 2S albumins from Brassica juncea (Braj 1E) and Sinapis alba L (Sin a 1). Ric c 1 and Ric c 3 may be classified as isoallergens if, additionally, the high degree of similarity in the position of polar residues is taken into account.