β-Lactoglobulin Self-Assembly: Structural Changes in Early Stages and Disulfide Bonding in Fibrils

Bovine beta-lactoglobulin (beta-Lg) self-assembles into long amyloid-like fibrils when heated at 80 C, pH 2, and low ionic strength (<0.015 mM). Heating beta-Lg under fibril-forming conditions shows a lag phase before fibrils start forming. We have investigated the structural characteristics of beta-Lg during the lag phase and the composition of beta-Lg fibrils after their separation using ultracentrifugation. During the lag phase, the circular dichroism spectra of heated beta-Lg showed rapid unfolding, and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of samples showed increasing hydrolysis of beta-Lg. The SDS-PAGE profiles of fibrils separated by ultra centrifugation showed that after six hours, the fibrils consisted of a few preferentially accumulated peptides. Two-dimensional SDS-PAGE under reducing and nonreducing conditions showed the presence of disulfide-bonded fragments in the fibrils. The sequences in these peptide bands were characterized by in-gel digestion electrospray ionization (ESI)-MS/MS. The composition of solubilized fibrils was also characterized by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS/MS. Both MS analyses showed that peptides in fibrils were primarily from the N-terminal region, although there was some evidence of peptides from the C-terminal part of the molecule present in the higher molecular weight gel bands. We suggest that although the N-terminal region of beta-Lg is almost certainly involved in the formation of the fibrils, other peptide fragments linked through disulfide bonds may also be present in the fibrils during self-assembly.