Amyloid fibrils and partially unfolded intermediates can be distinguished serologically from native amyloidogenic precursor proteins or peptides. In this regard, we previously had reported that mAb 11-1F4, generated by immunizing mice with a thermally denatured variable domain (V L ) fragment of the human κ4 Bence Jones protein Len, bound to a non-native conformational epitope located within the N-terminal 18 residues of fibrillar, as well as partially denatured, Ig light chains (O'Nuallain B. et al. (2006) Biochemistry 46, 1240. To define further the antibody binding site, we used random peptide phage display and epitope mapping of V L Len using wild-type and alaninemutated Len peptides where it was shown that the antibody epitope was reliant on up to 10 of the first 15 residues of protein Len. Comparison of V κ and V λ N-terminal germline consensus sequences with protein Len and 11-1F4-binding phages indicated that this antibody's cross-reactivity with light chains was related to an invariant proline at position(s) 7 and/or 8, bulky hydrophobic residues at positions 11 and 13, and additionally, to the ability to accommodate amino acid diversity at positions 1-4. Sequence alignments of the phage peptides revealed a central proline, often flanked by aromatic residues. Taken together, these results have provided evidence for the structural basis of the specificity of 11-1F4 for both κ and λ light chain fibrils. We posit that the associated binding site involves a rare type VI β-turn or touch-turn that is anchored by a cis-proline residue. The identification of an 11-1F4-related mimotope should facilitate development of pan-light chain fibril-reactive antibodies that could be used in the diagnosis and treatment of patients with AL amyloidosis.The amyloidoses are a group of over 20 protein misfolding disorders associated with often fatal sporadic and hereditary disorders, including Alzheimer's disease, type II diabetes, and primary (AL) amyloidosis (1-4). All types of amyloid, regardless of amino acid sequence, share tertiary structural and tinctorial features, i.e., they are formed from fibrils composed of extended β-sheets oriented perpendicularly to the long fibril axis (5,6) and bind the dyes thioflavin T and Congo red (7,8). Additionally, fibrils, as well as their assembly intermediates, contain generic conformational epitopes not present on the native proteins (9-16). These common characteristics reflect a similar fibrillogenic pathway(s) involving the assembly of soluble higher-order intermediates into fibrils that are structurally dissimilar to their precursors (2, 13,17).
