Unusual amino acid usage in the variable regions of mercury-binding antibodies

Monoclonal antibodies (mAb) specific for mercuric ions were isolated from B ALB/c mice injected with a mercury-containing, hapten-carrier complex. The antibodies reacted by enzyme-linked immunosorbent assay with bovine serum a lbumin-glutathione-mercuric chloride (BSA-GSH-HgCl) but not with BSA-GSH wi thout mercury. Nucleotide sequences from polymerase chain reaction products encoding six of the antibody heavy-chain variable regions and seven light- chain variable regions revealed that all the antibodies contained an unpair ed cysteine residue in one hypervariable region, which is unusual for murin e antibodies. Mutagenesis of the cysteine to either tyrosine or serine in o ne of the Hg-binding antibodies, mAb 4A10, eliminated mercury binding. Howe ver, of two influenza-specific antibodies that contain cysteine residues at the same position as mAb 4A10, one reacted with mercury, although not so s trongly as 4A10, whereas the other did not react at all. These results sugg ested that, in addition to an unpaired cysteine, there are other structural features, not yet identified, that are important for creating an appropria te environment for mercury binding. The antibodies described here could be useful for investigating mechanisms of metal-protein interactions and for c haracterizing antibody responses to structurally simple haptens. Proteins 1 999;37:429-440. (C) 1999 Wiley-Liss, Inc.