Structure-function analysis and insights into the reduced toxicity of Abrus precatorius agglutinin I in relation to abrin.
Abrin and agglutinin-I from the seeds of Abrus precatorius are type II ribosome-inactivating proteins that inhibit protein synthesis in eukaryotic cells. The two toxins share a high degree of sequence similarity; however, agglutinin-I is weaker in its activity. We compared the kinetics of protein synthesis inhibition by abrin and agglutinin-I in two different cell lines and found that approximately 200-2000-fold higher concentration of agglutinin-I is needed for the same degree of inhibition. Like abrin, agglutinin-I also induced apoptosis in the cells by triggering the intrinsic mitochondrial pathway, although at higher concentrations as compared with abrin. The reason for the decreased toxicity of agglutinin-I became apparent on the analysis of the crystal structure of agglutinin-I obtained by us in comparison with that of the reported structure of abrin. The overall protein folding of agglutinin-I is similar to that of abrin-a with a single disulfide bond holding the toxic A subunit and the lectin-like B-subunit together, constituting a heterodimer. However, there are significant differences in the secondary structural elements, mostly in the A chain. The substitution of Asn-200 in abrin-a with Pro-199 in agglutinin-I seems to be a major cause for the decreased toxicity of agglutinin-I. This perhaps is not a consequence of any kink formation by a proline residue in the helical segment, as reported by others earlier, but due to fewer interactions that proline can possibly have with the bound substrate.
Bagaria
Ashima
A
Department of Biochemistry, Indian Institute of Science, Bangalore 560012, India.
Surendranath
Kalpana
K
Ramagopal
Udupi A
UA
Ramakumar
Suryanarayanarao
S
Karande
Anjali A
AA
D000036:Abrin / Q000302:isolation & purification / Q000633:toxicity*; D029912:Abrus / Q000737:chemistry*; D017209:Apoptosis / Q000187:drug effects*; D001665:Binding Sites; D001943:Breast Neoplasms / Q000378:metabolism / Q000473:pathology; D018360:Crystallography, X-Ray; D006031:Glycosylation; D006801:Humans; D008958:Models, Molecular; D037121:Plant Lectins / Q000302:isolation & purification / Q000633:toxicity*; D011487:Protein Conformation; D011500:Protein Synthesis Inhibitors / Q000633:toxicity*; D012639:Seeds / Q000737:chemistry; D013329:Structure-Activity Relationship; D014407:Tumor Cells, Cultured
D037121:Plant Lectins; D011500:Protein Synthesis Inhibitors; D000036:Abrin