Epitope characterization of a supramolecular protein assembly with a collection of monoclonal antibodies: The case of casein micelle

In milk, κ-, β-, αs1- and αs2-casein (CN) are associated into a supramolecular assembly, the micelle. In this work, CN micelles contained in fresh skim milk were used to produce over 100 monoclonal antibodies. The specificity of these probes was determined using libraries of synthetic peptides and peptides fractionated from tryptic hydrolysis of purified CNs. Although κ-CN and αs2-CN are minor proteins in the micelle (ratio 1:1:4:4 for κ, αs2, αs1, β) a proportionally high number of clones were produced towards these two proteins (32 for each), compared to 9 and 29 for αs1-CN and β-CN, respectively. Most of the β-CN and κ-CN epitopes were identified, while about 50% of αs1-CN and αs2-CN antibodies were suspected to react to conformational linear or discontinuous epitopes, since no peptide binding could be identified. Antibody binding to the phosphoserine rich regions of the three calcium sensitive CNs was weak or non-existing, suggesting them to be hidden in the micelle structure together with αs1-CN. The C-terminal glycomacropeptide of κ-CN and the C-terminal moiety of β-CN were well exposed generating the majority of the antibodies specific for these two proteins. The two major antigenic sites of αs2 were αs2-CN (f96-114) and (f16-35). Cross-reaction between αs2-CN specific antibodies with αs1-CN illustrated the tangled structure between the two proteins. Immuno-dominant epitopes identified in the present study totally differ from those known for the purified caseins suggesting they were specific for the micelle supramolecular structure.