Early events of folding of β-structure-rich protein

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Protein folds via definite pathway. The purpose of the folding study is to find its folding pathway. We have developed cryo-stopped-flow apparatus with circular dichrosism, x-ray scattering and fluoresecence as probes. We have investigated various types of proteins. Results with alpha-helical protein show the folding rate of alpha-helix formation is so rapid that we cannot measure the rate of alpha-helix formation. In contrast, proteins, mainly consisting of beta-structure, take alpha-helix-rich intrmeditate at its earliest stage. We have focused the folding of SH3, which takes beta-conformation at its native state.SH3 takes alpha-helix-rich intermediate at its earliest stage of folding. We then investigated other beta-structure-rich proteins, and found the intermediate appeared without exception. We compared the amplitude of the burst with other parameters, and found the burst amplitude has good relationship with the helical fraction calculated by a program, Helix2. The program is mainly based on helix-coil transition theory with experimental data of adjacent residues. This suggest that the initial folding cores are formed by the interaction of short-lived alpha-helices.By the one residue mutation of SH3 (Alanine 45 to Glycine), we found the conformation changed from beta-rich to alpha-rich at pH3. This indicates the conversion of beta to alpha occurs more frequently than expected.