Interaction of phomopsin A and related compounds with purified sheep brain tubulin

Phomopsins comprise a family of peptide mycotoxins containing a 13-membered ring formed by an ether bridge, produced by the fungus Phomopsis leptostromiformis, the causal agent in lupin poisoning (lupinosis). The biochemical actions of two naturally occurring phomopsins, phomopsin A and B, and the chemical derivatives, phomopsinamine A and octahydrophomopsin A, on purified sheep brain tubulin were investigated. All analogues were potent microtubule inhibitors, blocking the polymerization of tubulin at concentrations of less than 1 microM. They inhibited [3H]vinblastine binding to tubulin and, in common with vinblastine and its competitive inhibitor maytansine, enhanced the binding of [3H]colchicine to tubulin. It is postulated that phomopsin A and its analogues exert their action on tubulin by interaction at or near the vinblastine binding site. Two possible mechanisms for the interaction between vinblastine or phomopsins and colchicine binding to tubulin are proposed.