= Solution
\b[Sequestration of assembly-competent <G-actin> explains both inhibition of <polymer> formation and loss of pre-existing cellular structures.] In a simple model, free <actin monomer> $G$ binds drug $D$ to form an assembly-incompetent complex $GD$. At a <actin filament> end the net addition rate is $v=k_+[G]-k_-$, so lowering free $[G]$ reduces elongation and can drive net shortening below the relevant <actin critical concentration> $C_c=k_-/k_+$. Formation of new nuclei is also strongly reduced when free <actin monomer> is depleted. Existing <actin filaments> can consequently disappear through ordinary turnover even though ToxB never binds them.
Binding could instead or additionally alter the <actin monomer> conformation or <nucleotide> state needed for productive assembly. The resistant mutant supports an interaction with <actin>, but resistance could reflect reduced drug affinity, an allosteric change or altered assembly properties. The available endpoints do not uniquely distinguish these possibilities. In particular, they provide no positive evidence for direct <actin filament severing> or <actin filament capping> by ToxB. A real example of actin-monomer sequestration and resistant-actin genetics is latrunculin A; this is an analogy, not a chemical identification of ToxB.<High rates of actin filament turnover in budding yeast and roles for actin in establishment and maintenance of cell polarity revealed using the actin inhibitor latrunculin-A by Kathryn R. Ayscough and others>[Purified-actin assembly and drug-resistant mutant analysis]{ref}
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