Epitope characterization and variable region sequence of f1-40, a high-affinity monoclonal antibody to botulinum neurotoxin type a (Hall strain).
BACKGROUND
Botulism, an often fatal neuroparalytic disease, is caused by botulinum neurotoxins (BoNT) which consist of a family of seven serotypes (A-H) produced by the anaerobic bacterium Clostridium botulinum. BoNT, considered the most potent biological toxin known, is a 150 kDa protein consisting of a 100 kDa heavy-chain (Hc) and a 50 kDa light-chain (Lc). F1-40 is a mouse-derived, IgG1 monoclonal antibody that binds the light chain of BoNT serotype A (BoNT/A) and is used in a sensitive immunoassay for toxin detection. We report the fine epitope mapping of F1-40 and the deduced amino acid sequence of the variable regions of the heavy and light chains of the antibody.


RESULTS
To characterize the binding epitope of F1-40, three complementary experimental approaches were selected. Firstly, recombinant peptide fragments of BoNT/A light-chain were used in Western blots to identify the epitope domains. Secondly, a peptide phage-display library was used to identify the specific amino acid sequences. Thirdly, the three-dimensional structure of BoNT/A was examined in silico, and the amino acid sequences determined from the phage-display studies were mapped onto the three-dimensional structure in order to visualize the epitope. F1-40 was found to bind a peptide fragment of BoNT/A, designated L1-3, which spans from T125 to L200. The motif QPDRS was identified by phage-display, and was mapped to a region within L1-3. When the three amino acids Q138, P139 and D140 were all mutated to glycine, binding of F1-40 to the recombinant BoNT/A light chain peptide was abolished. Q-138, P-139 and D-140 form a loop on the external surface of BoNT/A, exposed to solvent and accessible to F1-40 binding.


CONCLUSIONS
The epitope of F1-40 was localized to a single exposed loop (ss4, ss5) on the Lc of BoNT. Furthermore amino acids Q138, P139 and D140 forming the tip of the loop appear critical for binding.
Scotcher
Miles C
MC
United States Department of Agriculture, Agricultural Research Service, Western Regional Research Center, Albany, California, United States of America.
McGarvey
Jeffery A
JA
Johnson
Eric A
EA
Stanker
Larry H
LH
D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D000915:Antibody Affinity; D001483:Base Sequence; D015153:Blotting, Western; D019274:Botulinum Toxins, Type A / Q000276:immunology*; D017931:DNA Primers; D004591:Electrophoresis, Polyacrylamide Gel; D004797:Enzyme-Linked Immunosorbent Assay; D000939:Epitopes / Q000737:chemistry* / Q000276:immunology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010957:Plasmids; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000302:isolation & purification
D000911:Antibodies, Monoclonal; D017931:DNA Primers; D000939:Epitopes; D011993:Recombinant Fusion Proteins; D019274:Botulinum Toxins, Type A