Structures of the ribosome-inactivating protein from barley seeds reveal a unique activation mechanism

Ribosome-inactivating protein (RIP), a defence protein found in various plants, possesses different chain architectures and activation mechanisms. The RIP from barley (bRIP) is a type I RIP and has sequence features that are divergent from those of type I and type II RIPs from dicotyledonous plants and even the type III RIP from maize. This study presents the first crystal structure of an RIP from a cereal crop, barley, in free, AMP-­bound and adenine-bound states. For phasing, a codon-optimized synthetic brip1 gene was used and a vector was constructed to overexpress soluble bRIP fusion proteins; such expression has been verified in a number of cases. The overall structure of bRIP shows folding similar to that observed in other RIPs but also shows significant differences in specific regions, particularly in a switch region that undergoes a structural transition between a 3₁₀-helix and a loop depending on the liganded state. The switch region is in a position equivalent to that of a proteolytically susceptible and putative ribosome-binding site in type III RIPs. Thus, the bRIP structure confirms the detailed enzymatic mechanism of this N-­glycosidase and reveals a novel activation mechanism for type I RIPs from cereal crops.