Conformation similarities of ricin A-chain and trichosanthin

The conformation of ricin A-chain from castor bean was studied by circular dichroism at pH 4.7, 7 and 9 and compared with that of trichosanthin from the Chinese herb Tianhuafen. The CD spectra of ricin A-chain and trichosanthin were nearly identical at each of the three pHs. Analysis of the data indicated that, like trichosanthin, ricin A-chain had about 29% alpha-helix and 42% beta-sheet but no beta-turn. However, there was a subtle difference in the CD spectra in 20 mM sodium dodecyl sulfate, the addition of which at pH 7 slightly increased the helicity and decreased the content of beta-sheet of ricin A-chain in contrast to a larger increase in helicity at the expense of beta-sheet for trichosanthin, thus indicating a different stability against the surfactant. Native ricin A-chain and trichosanthin had about the same amount of secondary structure, which supports the belief that a high degree of sequence homology of the two proteins [Zhang & Wang (1986) Nature 321, 477-478] may lead to a conformational similarity between them, even though the two proteins are not taxonomically related.