Three-stage refolding/unfolding of the dual-color β-subunit in R-phycocyanin from Polysiphonia urceolata

The conformational changes during refolding and unfolding of the dual-color β-subunit in R-phycocyanin (R-PC) were monitored by the spectra, fluorescence anisotropy, and FRET. It was observed that both of the refolding and unfolding of the β-subunit would undergo a three-stage conformational change, but in a reverse order. During the refolding process, at the first stage, the configuration of the tetrapyrrole chromophores transformed from the cyclohelical to the extended one, suggested by the blue-shifted spectra. At the second stage, recovery of the hydrogen-bond and hydrophobic interaction network fixed the chromophore in a more rigid configuration, suggested by a linear increase in the total fluorescence yield. At the third stage, the increase of the FRET efficiency suggested a protein-framework movement that made the two chromophores closer or/and into a more parallel orientation. The fluorescence anisotropy further confirmed the three-stage model.