Ustiloxin A is a modified peptide derived from false smut balls on ric
e panicles, caused by the fungus Ustilaginoidea virens; structurally,
it resembles phomopsin A. Ustiloxin G is cytotoxic and is an inhibitor
of microtubule assembly in vitro. Because of its resemblance to phomo
psin A, we examined its interaction with tubulin and compared the resu
lts with those obtained with phomopsin A and dolastatin 10, both of wh
ich were found previously to have very similar effects. We determined
that ustiloxin A inhibited the formation of a particular intra-chain c
ross-link in beta-tubulin, as do vinblastine, maytansine, rhizoxin, ph
omopsin A, dolastatin 10, halichondrin B and homohalichondrin B; this
is in contrast to colchicine and podophyllotoxin which do not inhibit
formation of this cross-link. Ustiloxin A also inhibited the alkylatio
n of tubulin by iodo[C-14]acetamide, as do phomopsin A and dolastatin
10; vinblastine was almost as potent an inhibitor of alkylation as ust
iloxin A, whereas maytansine, halichondrin B and homohalichondrin B ha
ve little or no effect. In addition, ustiloxin A inhibited exposure of
hydrophobic areas on the surface of the tubulin molecule. In this res
pect, ustiloxin A was indistinguishable from phomopsin A but slightly
more effective than dolastatin 10 and considerably more effective than
vinblastine; this provides a strong contrast to maytansine, rhizoxin,
and homohalichondrin B which have no effect on exposure of hydrophobi
c areas and to halichondrin B which enhances exposure. Lastly, ustilox
in A strongly stabilized the binding of [H-3]colchicine to tubulin. Th
e combination of ustiloxin A with colchicine stabilized tubulin with a
half-life of over 8 days, comparable with results obtained with phomo
psin A and colchicine. A comparison of the structures of ustiloxin A,
phomopsin A and dolastatin 10 raised the possibility that the strong s
tabilization of the tubulin structure may require a short segment of h
ydrophobic amino acids such as the modified valine-isoleucine sequence
present in all three compounds. The rest of the structure, specifical
ly the large ring of ustiloxin A and phomopsin A, may serve to place t
his sequence in an appropriate conformation to interact with tubulin.