β-Casein will chaperone β-lactoglobulin during nanofibril assembly, but prefers familiar company at high concentrations

Bovine beta-casein behaves as a chaperone at neutral pH, i.e., it inhibits aggregation of unfolded proteins, but chaperone effects have not been investigated at acidic pH. We examined chaperone effects during heating at pH 2 and 80 degrees C for mixtures of 1% (w/v) beta-lactoglobulin (beta-Lg) and beta-Lg: beta-casein molar ratios of 1: 0.0625 to 1:1. These conditions hydrolyse beta-Lg into peptides that self-assemble into amyloid-like nanofibrils. Hydrolysis rates in mixed solutions were unchanged from single-protein controls, according to sodium dodecyl sulphate polyacrylamide gel electrophoresis. At ratios >= 1:0.125, beta-casein slowed fibril growth in a concentration-independent way, according to Thioflavin T fluorescence. We observed twisted irregular fibrils coexisting with long semiflexible fibrils when beta-casein was heated with beta-Lg, using transmission electron microscopy. We hypothesise that beta-casein monomers and peptides can interact with growing beta-Lg nanofibrils or assemble into micelles, with the latter pathway predominating at high beta-casei n concentrations. (C) 2016 Elsevier Ltd. All rights reserved.