Folding is coupled to dimerization of Tctex-1 dynein light chain

Equilibrium analyses have been performed to elucidate the role of dimerization in folding and stability of dynein light chain Tctex-1. The equilibrium unfolding transition was monitored by intrinsic fluorescence intensity, fluorescence anisotropy, and circular dichroism and was modeled as a two-state mechanism where a folded dimer dissociates to two unfolded monomers without populating thermodynamically stable monomeric or dimeric intermediates. Sedimentation equilibrium and chemical cross-linking experiments performed at increasing concentrations of denaturants show no change in the association state before the unfolding transition and are consistent with the two-state model of dissociation coupled to unfolding. A linear dependence on denaturant concentration is observed by fluorescence intensity and anisotropy before unfolding in the 0-2 M GdnCl, and 0-4 M urea concentration range. This change is not protein concentration-dependent and possibly reflects relief of quenching associated with premelting conformational disorder in the vicinity of Trp 83. The data clearly show that the dissociation-coupled unfolding mechanism of Tctex-1 is different from the three-state denaturation mechanism of its structural homologue light chain LC8. The absence of a stable monomer in Tctex-1 offers insight into its functional differences from LC8.