Extreme Sequence Divergence but Conserved Ligand-Binding Specificity in Streptococcus pyogenes M Protein

• Published in: PLoS pathogens Vol. 2; no. 5; p. e47
• Main Authors: Persson, Jenny, Beall, Bernard, Linse, Sara, Lindahl, Gunnar
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 01.05.2006; Public Library of Science (PLoS)
• Subjects: Amino Acid Sequence; Antigens - biosynthesis; Bacteria; Bacterial Outer Membrane Proteins - genetics; Bacterial Outer Membrane Proteins - immunology; Bacterial Outer Membrane Proteins - metabolism; Bacteriology; Basic Medicine; Chemical properties; Complement C4b-Binding Protein - metabolism; Complementarity Determining Regions; Eubacteria; Evolution; Genetics; Humans; Immunology; Infectious Diseases; Ligands; Logos; Medical and Health Sciences; Medicin och hälsovetenskap; Medicinska och farmaceutiska grundvetenskaper; Microbiology; Microbiology in the Medical Area; Microorganisms; Mikrobiologi inom det medicinska området; Molecular Biology - Structural Biology; Molecular Sequence Data; Mutagenesis, Site-Directed; Protein folding; Protein Structure, Tertiary; Residues; Streptococcus pyogenes; Streptococcus pyogenes - metabolism; Sweden
• ISSN: 1553-7374; 1553-7366; 1553-7374
• DOI: 10.1371/journal.ppat.0020047

Many pathogenic microorganisms evade host immunity through extensive sequence variability in a protein region targeted by protective antibodies. In spite of the sequence variability, a variable region commonly retains an important ligand-binding function, reflected in the presence of a highly conserved sequence motif. Here, we analyze the limits of sequence divergence in a ligand-binding region by characterizing the hypervariable region (HVR) of Streptococcus pyogenes M protein. Our studies were focused on HVRs that bind the human complement regulator C4b-binding protein (C4BP), a ligand that confers phagocytosis resistance. A previous comparison of C4BP-binding HVRs identified residue identities that could be part of a binding motif, but the extended analysis reported here shows that no residue identities remain when additional C4BP-binding HVRs are included. Characterization of the HVR in the M22 protein indicated that two relatively conserved Leu residues are essential for C4BP binding, but these residues are probably core residues in a coiled-coil, implying that they do not directly contribute to binding. In contrast, substitution of either of two relatively conserved Glu residues, predicted to be solvent-exposed, had no effect on C4BP binding, although each of these changes had a major effect on the antigenic properties of the HVR. Together, these findings show that HVRs of M proteins have an extraordinary capacity for sequence divergence and antigenic variability while retaining a specific ligand-binding function.