J. Biol. Chem., Vol. 268, Issue 31, 23000-23007, 11, 1993
Effect of heavy chain signal peptide mutations and NH2-terminal chain length on binding of anti-digoxin antibodies
J Ping, JF Schildbach, SY Shaw, T Quertermous, J Novotny, R Bruccoleri and MN Margolies
Department of Surgery, Massachusetts General Hospital, Boston 02114.
In certain instances, antibody variable region mutations outside of the
antigen-combining site influence antigen binding. We reported previously
that a heavy chain mutation (Ser-94-->Arg) decreased binding of the
anti-digoxin antibody 40-150, whereas an additional signal peptide mutation
at the -2 position (Gln-->Pro) causing NH2-terminal 2- residue
truncation partially restored binding. To assess the combined effects on
binding of two seemingly distant mutations, we constructed signal peptide
mutations and NH2-terminal deletions in the presence of Ser-94 and Arg-94.
Deletions of one to three amino acids had little effect on binding for
Ser-94 mutants, whereas 2-residue truncations produced directly or by
signal peptide mutation increased affinity approximately 40-fold for Arg-94
mutants. These observations are consistent with the reported
computer-generated model of antibody 40- 150. Introduction of Pro at the
signal peptide -3 position in 40-150 resulted in cleavage at alternative
sites, with varying effects on affinity. Introduction of Pro at -2 into the
anti-digoxin antibody 26- 10 resulted, unexpectedly, in expression of heavy
chains with 3 extra NH2-terminal residues, causing an approximately
100-fold reduction in affinity. Thus, both extensions and deletions of the
heavy chain amino terminus can enhance or reduce antigen binding, depending
on the structural context of specific antigen combining sites.