Microenvironment of cysteine 242 in type-1 ribosome-inactivating protein from iris

IRIP is a type-1 ribosome-inactivating protein isolated from the bulbs of I ris hollandica, It is one of the few type-1 RIPs that contain Cys residue(s ) in their primary sequence. IRIP contains a single Cys residue at position 242. Although IRIP is thought to be a monomeric protein, SDS-PAGE indicate s that part of the IRIP molecules can exist as disulphide bridge-linked dim ers, Probing of the reactivity of the unique Cys residue by 5,5'-dithiobis( 2-nitrobenzoic acid) indicates that Cys(242) in IRIP is free but is only pa rtially accessible to modifiers. Molecular modelling of IRIP is in agreemen t with this conclusion, Binding of the ligands adenine and poly(A) results in little or no effect on the conformation of Cys(242) in IRIP. Chemical mo dification of IRIP by a specific thiol modifier does not abolish the RNA N- glycosidase activity of IRIP, suggesting that Cys(242) is not critical for the enzymatic activity of IRIP. These results suggest that IRIP has the pot ential to be developed as a novel immunotoxin, (C) 2000 Academic Press.