Circular Dichroism and Fluorescence Studies of Polyomavirus Major Capsid Protein VP1

The conformational changes of polymavirus (Py) major capsid protein VP₁ in solution by the solution pH, addition of calcium, and ionic strength were examined by circular dichroism (CD) and fluorescence spectroscopy. Comparison of the predicted secondary structures of PyVP₁ and simian virus (SV) 40 by the methods of Chou-Fasman, Gamier et al., and Yang method are presented. Hydropathicity, surface probability, and chain flexibility of PyVP₁ were computer-analyzed by the methods of Kyte and Doolittle, Emini et al., and Karplus and Schulz, respectively. The CD measurements indicate that the secondary structure of PyVP₁ is little dependent on its concentration, Ca²⁺ concentration, and ionic strength, but is strongly pH dependent. Fluorescence studies showed that emission spectra of PyVP₁ are also pH-dependent. At extreme acidic and alkaline pH, the fluorescence intensity of PyVP₁ is decreased and the emission maximum is red-shifted. The fluorescence of PyVP₁ is quenched by the presence of CsCl, KI, and acrylamide. The analyses of the modified Stern–Volmer plots indicate that five of seven tryptophan residues in PyVP₁ are located on the surface of the protein, among which two are accessible to Cs⁺ and the other three are accessible to I^?. The two others are buried more deeply in the interior of the protein molecule.