Interaction of phomopsin A with normal and subtilisin-treated bovine brain tubulin

Tubulin, the major component of microtubules, has a tendency to lose its ability to assemble or to bind to ligands in a time-dependent process known as 'decay'. The decay process also causes tubulin to expose sulfhydryl groups and hydrophobic areas. The antimitotic drug phomopsin A strongly protects the tubulin molecule from decay. Here we have studied the interaction of phomopsin A with αβ tubulin and tubulin which has been treated with subtilisin to remove selectively the C-termini of the α and β chains (α(s)β(s)). The binding of phomopsin A to αβ tubulin decreases the sulfhydryl titer by approximately 1.0 mol/mol. Selective removal of the peptides from the C-terminal ends does not affect phomopsin A's interaction with tubulin. Moreover, the α(s)β(s) tubulin-phomopsin A complex appears to be more stable than the αβ tubulin-phomopsin A complex as determined by the time-dependent increase in exposure of sulfhydryl groups and hydrophobic areas on tubulin. In fact, phomopsin A inhibits the decay process of α(s)β(s) tubulin completely. This observation raises the possibility of determining the conformation of this configuration of tubulin.