A monoclonal antibody enhances ligand binding of a fibronectin MSCRAMM (adhesin) from Streptococcus dysgalactiae

A monoclonal antibody 3A10, generated from a mouse
immunized with the Streptococcus dysgalactiae fibronectin
(Fn) binding protein FnbA, was isolated, and
its effect on ligand binding by the antigen was examined.
The epitope for 3A10 was localized to a previously
unidentified Fn binding motif (designated Au) just Nterminal
of the repeat domain which represents the primary
ligand binding site on FnbA. Fn binding to Au was
enhanced by 3A10 rather than inhibited. This effect was
demonstrated in two different assays. First, in the presence
of 3A10 the Au-containing proteins and synthetic
peptide more effectively competed with bacterial cells
for binding to Fn. Second, 3A10 dramatically increased
the binding of biotin-labeled forms of the Au-containing
proteins to Fn immobilized on a blotting membrane.
Pure 3A10 IgG did not recognize the antigen by itself,
and Fn was required for the immunological interaction
between the antibody and the epitope. This induction
effect of Fn was shown in both Western blot and enzymelinked
immunosorbent assay in which immobilized Aucontaining
molecules were probed with 3A10 in the
presence of varying concentrations of Fn. Specificity
analyses of 3A10 revealed that the monoclonal also recognized
a ligand binding motif in a Streptococcus pyogenes
Fn binding MSCRAMM but not the corresponding
motifs in two related adhesins from Staphylococcus aureus
and S. dysgalactiae. Furthermore, 3A10 stimulated
Fn binding by S. pyogenes cells. These results together
with subsequent biophysical studies presented in the
accompanying paper (House-Pomepeo, K., Xu, Y., Joh,
D., Speziale, P., and Ho¨o¨ k, M. (1996) J. Biol. Chem. 271,
1379–1384) indicate that the ligand binding sites of Fn
binding MSCRAMMs have little or no secondary structure.
However, on binding to Fn, they appear to undergo
a structural rearrangement resulting in a defined structure
rich in b sheet and expressing a ligand-induced
binding site for antibodies such as 3A10.