Ricin A Chain from Ricinus sanguineus: DNA sequence, structure and toxicity.

The gene coding for the ricin A-chain from Ricinus sanguineus (RsTA) was isolated and sequenced (GB: DQ661048). Comparison of RsTA with the ricin A-chain from Ricinus communis (RTA) revealed the presence of five differences in the gene sequence. At the protein level only two differences were noticed, the replacement of Asn136 by Ser (N136S) and Ile173 by Val (I174 V). From the RsTA structure model (PMDB: PM0074652), the N136S mutation was predicted to have no effect on RsTA structure. The I174V mutation is believed to have no effect on the RsTA structure or on toxicity since this replacement was found in the Ricinus agglutinin's A-chain and the latter has a comparable toxicity to RTA. The Ser221 of the putative lipase active site was found. Toxicity experiments showed that RsTA and RTA have similar toxicities. This finding proves that the N136S and I174 V mutations have no effect on RsTA toxicity.