Structure/function studies on two type 1 ribosome inactivating proteins: Bouganin and lychnin

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• Ribosome-inactivating proteins (RIPs) are enzymes that inhibit protein synthesis.
• Type 1 RIPs, such as bouganin and lychnin, share structural similarities but exhibit varying activities.
• Understanding the structure-activity relationship of RIPs is crucial for developing targeted therapeutics.

Purpose of the Study:

• To determine the three-dimensional structures of bouganin and lychnin.
• To compare the adenine polynucleotide glycosylase activity of various RIPs.
• To elucidate the structural basis for the high activity of saporin-S6.

Main Methods:

• X-ray crystallography for structure determination.
• Enzymatic assays to measure adenine release from different polynucleotides (ribosomes, poly(A), DNA).
• Protein synthesis inhibition assays.
• Structural superposition and analysis of electrostatic surface potential.

Main Results:

• The 3D structures of bouganin and lychnin were solved, revealing conserved RIP fold with domain differences.
• Saporin-S6 showed the highest adenine release from rat ribosomes and poly(A).
• Saporin-S6's efficiency on herring sperm DNA was comparable to dianthin 30, bouganin, and PAP-R.
• Saporin-S6 demonstrated the highest protein synthesis inhibitory activity.
• Structural analysis identified a unique combination of negative electrostatic potential and positive charges at saporin-S6's active site.

Conclusions:

• The conserved RIP fold is maintained in bouganin and lychnin.
• Saporin-S6 possesses unique structural features at its active site, contributing to its potent enzymatic activity.
• These structural characteristics facilitate efficient substrate interaction and catalysis, explaining its high efficacy compared to other RIPs.