Characterization of victorin C, the major host-selective toxin fromCochliobolus victoriae: Structure of degradation products

Abstract

Several host-selective toxins have been isolated in pure form from culture filtrates of Cochliobolus victoriae.Acid hydrolysis of the major toxin, victorin C (apparent mol.wt 796), produced five fragments to which structures 1-5 have been assigned. Spectroscopic techniques revealed that the toxin contains an additional subunit corresponding to 6; thus all the components of victorin C are accounted for.

Molecular Features Affecting the Biological Activity of the Host-Selective Toxins from Cochliobolus victoriae

“…However, a single experiment showed that it was at least as active as victorin C, 1, from which it differs by an additional chlorine atom in the N-terminal amino acid. Introduction of a six-membered aromatic ring in the Cterminal amino acid residue and removal of two Cl atoms from the N-terminal amino acid residue as in victoricine, 5, reduces the activity of the molecule without affecting its selectivity. Because the biological activity seems to be insensitive to the number of Cl atoms in the N-terminal leucine unit, as judged by comparison of 1 with 2 and 4, the observed reduction of biological activity in victoricine, 5, is likely to be caused largely, if not exclusively, by the change in the C-terminal residue.…”

“…Introduction of a six-membered aromatic ring in the Cterminal amino acid residue and removal of two Cl atoms from the N-terminal amino acid residue as in victoricine, 5, reduces the activity of the molecule without affecting its selectivity. Because the biological activity seems to be insensitive to the number of Cl atoms in the N-terminal leucine unit, as judged by comparison of 1 with 2 and 4, the observed reduction of biological activity in victoricine, 5, is likely to be caused largely, if not exclusively, by the change in the C-terminal residue. In fact, comparison of the activities of 1, 3, and 5 suggest that the nature and the substitution pattern on the 12-membered macrocyclic ring is important in the association of the toxin with the host's active-site.…”

“…Recently (5,13,14), we have isolated and determined the structure offive naturally occurring forms ofthe toxin designated victorin B, C, D, E, and victoricine, which are closely related, highly active peptides containing an unusual 12-membered ring. The predominant form in culture ffiltrates is victorin C, which accounts for 85 to 90% of the total activity and inhibits root growth of susceptible seedlings by 50% at 80 to 120 pg/ml (5).…”

“…The predominant form in culture ffiltrates is victorin C, which accounts for 85 to 90% of the total activity and inhibits root growth of susceptible seedlings by 50% at 80 to 120 pg/ml (5). An additional toxin, victorin M, was isolated and its structure determined by Kono et al (4).…”

“…MATERIALS AND METHODS The production and purification of victorin C (1), victorins B, D, and E (24) and victoricine (5) (Table I) have been described previously (5). All toxin derivatives were purified by CI8 reversedphase column chromatography (Z module ,Bondapak, Waters Assoc.)…”