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, Garnier 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.