Stable folding intermediates prevent fast interconversion between the closed and open states of Mad2 through its denatured state.

Different states of metamorphic proteins can interconvert under physiological conditions to realize corresponding functions. The mechanism behind the conversion is critical for understanding how these proteins work. We report a combined thermodynamic and kinetic study on the folding/unfolding process of the open and closed conformers of mitotic arrest deficient protein 2 (Mad2), a metamorphic protein. It has been observed that open Mad2 (O-Mad2) can convert to closed Mad2 (C-Mad2). Our results show that O-Mad2 and C-Mad2 have similar thermodynamic stability, which explains the presence of metamorphosis. The folding/unfolding kinetics suggest that the conversion between O-Mad2 and C-Mad2 would be much faster than that reported previously if this conversion goes through the denatured state (U) directly, i.e. through an O-Mad2-denatured state (U)-C-Mad2 (O-U-C) pathway. This inconsistency implies that there exist stable intermediates in between the native and denatured states of Mad2, which would either slow down the O-U-C interconversion or prevent it going through the denatured state.