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Departments of
*
Medical Microbiology, Dermatology, and Infection and
Physical Chemistry 2, Lund University, Lund, Sweden; and
Department of Theoretical Physics, Royal Institute of Technology, Stockholm, Sweden
Antigenic variation in microbial surface proteins represents an
apparent paradox, because the variable region must retain an important
function, while exhibiting extensive immunological variability. We
studied this problem for a group of streptococcal M proteins in which
the 
50-residue hypervariable regions (HVRs) show essentially no
residue identity but nevertheless bind the same ligand, the human
complement regulator C4b-binding protein (C4BP). Synthetic peptides
derived from different HVRs were found to retain the ability to bind
C4BP, implying that the HVR corresponds to a distinct ligand-binding
domain that can be studied in isolated form. This finding allowed
direct characterization of the ligand-binding properties of isolated
HVRs and permitted comparisons between different HVRs in the absence of
conserved parts of the M proteins. Affinity chromatography of human
serum on immobilized peptides showed that they bound C4BP with high
specificity and inhibition experiments indicated that different
peptides bound to the same site in C4BP. Different C4BP-binding
peptides did not exhibit any immunological cross-reactivity, but
structural analysis suggested that they have similar folds. These data
show that the HVR of streptococcal M protein can exhibit extreme
variability in sequence and immunological properties while retaining a
highly specific ligand-binding function.
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