Isolated hypervariable regions derived from streptococcal M proteins specifically bind human C4b-binding protein: Implications for antigenic variation

Antigenic variation in microbial surface proteins represents an apparent pa radox, because the variable region must retain an important function, while exhibiting extensive immunological variability. We studied this problem fo r a group of streptococcal M proteins in which the similar to 50-residue hy pervariable regions (HVRs) show essentially no residue identity but neverth eless bind the same ligand, the human complement regulator C4b-binding prot ein (C4BP). Synthetic peptides derived from different HVRs were found to re tain the ability to bind C4BP, implying that the HVR corresponds to a disti nct ligand-binding domain that can be studied in isolated form. This findin g allowed direct characterization of the ligand-binding properties of isola ted HVRs and permitted comparisons between different HV Rs in the absence o f 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 s ame site in C4BP. Different C4BP-binding peptides did not exhibit any immun ological 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.