Effect of α-domain substitution on the structure, property and function of human neuronal growth inhibitory factor

Human metallothionein-3 (hMT3), also named human neuronal growth inhibitory factor (hGIF), is attractive due to its distinct neuronal growth inhibitory activity, which is not shown by other human MT isoforms. It has been reported that the neuronal growth inhibitory activity arises from the N-terminal β-domain rather than its C-terminal α-domain. However, previous bioassay results have shown that the single β-domain is less effective at inhibiting the neuron growth than that in intact hMT3 on a molar basis, which suggests that the α-domain is indispensable to the neuronal growth inhibitory activity of hMT3. In order to confirm this assumption, we constructed two domain-hybrid mutants, the β(MT3)-β(MT3) mutant and the β(MT3)-α(MT1) mutant, and investigated their structural and metal binding properties by UV-vis spectroscopy, CD spectroscopy, pH titration, DTNB reaction, EDTA reaction, etc. The results showed that stability of the Cd ₃S₉ cluster of the β(MT3)-β(MT3) mutant decreased significantly while the Cd₃S₉ cluster of the β(MT3)-α(MT1) mutant had a similar stability and solvent accessibility to that of hMT3. Interestingly, the bioassay results showed that the neuronal growth inhibitory activity of the β(MT3)-β(MT3) mutant decreased significantly, while the β(MT3)-α(MT1) mutant showed similar inhibitory activity to hMT3. Based on these results, we conclude that the α-domain is indispensable and plays an important role in modulating the stability of the metal cluster in the β-domain by domain-domain interactions, thus influencing the bioactivity of hMT3.