Complete amino-acid sequence of PD-S2, a new ribosome-inactivating protein from seeds of Phytolacca dioica L

The primary structure has been determined for PD-S2, a new type 1 ribosome-inactivating protein (RIP), isolated from the seeds of Phytolacca dioica L. PD-S2 has 265 amino-acid residues, and a molecular mass of 29 586 Da. The polypeptide chain contains four amino-acid residues more than PAP-S, a type-1 RIP isolated from the seeds of the taxonomically related plant Phytolacca americana L. We have compared the amino-acid sequence of PD-S2 with those of two other RIPs with known three-dimensional structure: PAP-S and ricin A-chain (RTA), the active chain of the best known type-2 RIP. This analysis shows an identity of 76% and 33% with PAP-S and RTA respectively, and a similarity of 82% and 54%. Comparison with the PAP sequence, isolated from leaves of P. americana, shows an even higher identity (80%) and similarity (87%). Furthermore, the amino-acid residues reported in other RIPs to be invariant and participate in the definition of the active site (Tyr-76, Tyr-127, Glu-179, Arg-182 and Trp-211; PD-S2 numbering) are all present, Asn-74, Arg-138, Gln-175, and Glu-208 are also conserved, while Asn-309 is substituted by Glu, all residues located in the active-site cleft of RIPs (Tahirov, T.H., Lu, T.-H.: Liaw, Y.-C., Chen, J.L. and Lin, J.Y, (1995) Crystal structure of abrin-a at 2.14 Angstrom. J. Mel. Biol. 250, 354-367). The polypeptide chain of PD-S2 contains two N-glycosylation sites at Asn-112 and Asn-120, the second of which appears to be linked to sugars. Like PAP-S, PD-S2 does not contain free sulfhydryl groups. The four cysteinyl residues of the two proteins have corresponding sequence positions, most likely with identical S-S pairing.