Structural Characteristic of Folding/Unfolding Intermediate of Pokeweed Anti-viral Protein Revealed by Time-resolved Fluorescence

The structural feature of unfolding intermediate of pokeweed anti-viral protein (PAP) was characterized using time-resolved fluorescence spectroscopic methods to elucidate protein folding/unfolding process. CD and fluorescence spectra consistently demonstrated that the unfolding of PAP completed at 4 M of guanidine hydrochloride (GuHCl). The fluorescence resonance energy transfer (FRET) and time-resolve fluorescence depolarization analysis of Trp208 and Trp237 located in the C-terminal domain of PAP suggested that peculiar unfolding intermediate populated before reaching to the unfolding state. The FRET distance of Trp237 to Tyr182 was extended to more than 28 Å with keeping the compact conformation in the unfolding intermediate state populated in the presence of 2 M GuHCl. On the other hand, Trp208 maintained the energy transfer pair with Tyr72 near the active site, although the rotational freedom was increased a little. There results suggest that the most distinguished structural feature of the unfolding intermediate of PAP is the separation of C-terminal domain from N-terminal domain. FRET and fluorescence depolarization studies also showed that C-terminal domain would be more separated to liberate the segmental motions of Trp208 and Trp237 distinctly at the unfolding state.