Characterisation of the yam tuber storage protein dioscorin

Dioscorin (M(r) 31,000) is the major soluble protein present in the tubers of the tropical root crop yam (Dioscorea spp.) and in view of its abundance is considered to act as a storage protein. Because of the importance of the tuber as food for human nutrition the characterisation of dioscorin from Dioscorea cayenensis was undertaken to evaluate the possibility of crop improvement through genetic modification. Six charge isomers were detected by IEF, with pIs in the range 5.68-6.38, and 2-D non-reduced/reduced SDS-PAGE showed two major groups of components differing in the presence or absence of intra-chain disulphide bonds. The partial amino acid sequence determined from the protein N-terminus and a V8 protease cleavage product agreed with the sequences deduced from the nucleotide sequences of cDNAs. Circular dichroism spectroscopy showed a secondary structure rich in α-helix which agreed with secondary structure predictions based on the protein sequence. A significant degree of sequence similarity was found between dioscorins and α-carbonic anhydrase enzymes from animals. However, dioscorin may not be an active enzyme due to the presence of amino acid substitutions in the region corresponding to the carbonic anhydrase active site. Immuno-localisation experiments showed that dioscorin accumulates specifically in the vacuoles of tuber cells, as reported for the tuber storage proteins of potato and sweet potato.