Characterization and molecular cloning of two different type 2 ribosome-inactivating proteins from the monocotyledonous plant Polygonatum multiflorum

Leaves of the monocotyledonous plant Polygonatum multiflorum L. (Solomon's seal) contain besides a monocot mannose-binding lectin two galactose/N-acet ylgalactosamine (Gal/GalNAc)-binding type 2 ribosome-inactivating proteins (RIPs). Both RIPs were purified using a combination of classical protein pu rification techniques and affinity chromatography. Although both RIPs consi st of protomers of 65 kDa, the P. multiflorum RIP monomer (PMRIPm) occurs a s a monomer of approximately 60 kDa, whereas the tetramer (PMRIPt) is a tet ramer of 240 kDa. Both RIPs exhibit similar RNA N-glycosidase activity but differ in their specific agglutination activity and carbohydrate-binding sp ecificity, PMRIPt being a GalNAc-specific lectin whereas PMRIPm is Gal/GalN Ac-specific. Toxicity tests indicated that both Polygonatum RIPs exhibit a very low cytotoxicity towards human and animal cells. Analysis of the genom ic clones encoding both RIPs revealed a high degree of sequence similarity to other type 2 RIPs. Molecular modelling confirmed that both Polygonatum R IPs have a similar structure to ricin.