The Ricinus communis 2S albumin precursor: a single preproprotein may be processed into two different heterodimeric storage proteins.

The Ricinus communis (castor bean) 2S albumin is a heterodimer of glutamine-rich, disulphide-linked 4 and 7 kDa polypeptide. A cDNA library was constructed using mRNA from maturing castor bean endosperm as template. Clones containing sequences complementary to albumin mRNA were isolated by hybridization using as a probe a mixture of synthetic oligonucleotides representing sequences predicted for a peptide present in the 2S albumin large subunit. The nucleotide sequence contained an open reading frame encoding a preproprotein of 258 amino acid residues. The preproprotein included both polypeptides of the previously sequenced 2S albumin. In addition, this precursor included two further glutamine-rich sequences which, in term of their size and conserved cystein residues typically found in seed proteins of the 2S albumin superfamily, possible represent the small and large polypeptide subunits of a second heterodimeric storage protein. A post-translational processing scheme is proposed which would result in a single preproprotein generating two distinct heterodimeric 2S albumins. The generation of a second heterodimer seems likely since polypeptide candidates for its small and large subunits were found in the Ricinus 2S albumin fraction, and N-terminal protein sequencing confirmed the existence of the putative small subunit.