Zn ²⁺ can bind microtubules and inhibits motor protein activity in vitro. (A and B) Microtubule-stimulated ATPase activity of (A) purified recombinant human kinesin (KIF5a), and (B) a minimally processive, artificially dimerized yeast dynein fragment (GST-dynein₃₃₁) across a range of precisely buffered Zn²⁺ concentrations (see Materials and methods), normalized to the maximum and minimum ATPase activity within each individual replicate, replicates denoted by separate colors and marker shapes (kinesin, n = 4 individual replicates; dynein, n = 3 individual replicates). Each assay was performed in the presence of 2 µM microtubules. Black line shows cumulative data fitted to cubic model with log transformation of [Zn²⁺] concentration, with corresponding R² values shown in the top right corner. (C) Mean normalized microtubule-stimulated ATPase activity (±SEM) for a minimally processive, artificially dimerized yeast dynein fragment (GST-dynein₃₃₁, purple) and recombinant human kinesin (KIF5A, blue) in the presence of 500 µM CaCl₂ (n = 3 replicates each). (D) Mean (±SEM) in vitro fluorescent intensity (488 nm excitation, 525 nm emission) for solutions containing 1 µM FluoZin-3, 1 µM ZnCl₂, 1 mM DTT, and varied concentrations of pre-formed, taxol-stabilized porcine microtubules (MT; n = 4 individual replicates). (E and F) Traditional turbidity assay of microtubule polymerization across a range of precisely buffered Zn²⁺ concentrations (see Materials and methods). (E) Mean absorbance (±SEM) at 340 nm and (F) maximum polymerization velocity for each Zn²⁺ concentration (n = 3 replicates per Zn²⁺ concentration). **** P < 0.0001, n.s. not significant.