Folding and stability studies on C-PE and its natural N-terminal truncant

The conformational and functional state of biliproteins can be determined by optical properties of the covalently linked chromophores. α-Subunit of most of the phycoerythrin contains 164 residues. Recently determined crystal structure of the naturally truncated form of α-subunit of cyanobacterial phycoerythrin (Tr-αC-PE) lacks 31 N-terminal residues present in its full length form (FL-αC-PE). This provides an opportunity to investigate the structure–function relationship between these two natural forms. We measured guanidinium chloride (GdmCl)-induced denaturation curves of FL-αC-PE and Tr-αC-PE proteins, followed by observing changes in absorbance at 565 nm, fluorescence at 350 and 573 nm, and circular dichroism at 222 nm. The denaturation curve of each protein was analyzed for Δ G D ∘ , the value of Gibbs free energy change on denaturation (ΔGD) in the absence of GdmCl. The main conclusions of the this study are: (i) GdmCl-induced denaturation (native state ↔ denatured state) of FL-αC-PE and Tr-αC-PE is reversible and follows a two-state mechanism, (ii) FL-αC-PE is 1.4 kcal mol−1 more stable than Tr-αC-PE, (iii) truncation of 31-residue long fragment that contains two α-helices, does not alter the 3-D structure of the remaining protein polypeptide chain, protein–chromophore interaction, and (iv) amino acid sequence of Tr-αC-PE determines the functional structure of the phycoerythrin.