Identification and analysis of peptide determinants of murine monoclonal anti-Sm B/B' autoantibodies
The peptides Sm B and B' are components of the spliceosome which are commonly bound by autoantibodies in systemic lupus erythematosus in man and in MRL lpr/lpr mice. We have characterized anti-Sm B/B' murine monoclonal autoantibodies. Sequence analysis of the heavy chain variable region of the monoclonals reveals likely somatic mutation of the V23 germline gene indicative of a mature B-cell response. The linear antigenic regions of these monoclonals have been mapped using overlapping octapeptides. Monoclonal antibody KSm 5 recognizes the peptide PPPGMRPP which is repeated three times in the Sm B' polypeptide. KSm 3 binds two very similar, almost neighboring octapeptides PPPGIRGP and PGIRGPPP. Amino acid substitution and deletion experiments revealed that the arginine in these peptides is critical to antibody recognition. KSm 5 recognized PPPGMxPP peptides (x = A, G, R, or S). In contrast, substitution of the arginine in the PPPGIRGP peptide abolished KSm 3 binding. These results and molecular dynamic modeling suggest that binding of KSm 5 is mediated through backbone interactions rather than charge or hydrophobicity of the substituted amino acid, whereas recognition by KSm 3 is dependent upon the arginine side chain. Thus, autoantibodies may be capable of binding quite different aspects of similar peptide structures.
McClain, MT; Kaufman, KM; Koelsch, G; Harley, JB; James, JA
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