Streptococcal M protein: structural studies of the hypervariable region, free and bound to human C4BP.
Streptococcus pyogenes is a Gram-positive bacterium that causes several diseases, including acute tonsillitis and toxic shock syndrome. The surface-localized M protein, which is the most extensively studied virulence factor of S. pyogenes, has an approximately 50-residue N-terminal hypervariable region (HVR) that plays a key role in the escape of the host immunity. Despite the extensive sequence variability in this region, many HVRs specifically bind human C4b-binding protein (C4BP), a plasma protein that inhibits complement activation. Although the more conserved parts of M protein are known to have dimeric coiled-coil structure, it is unclear whether the HVR also is a coiled coil. Here, we use nuclear magnetic resonance (NMR) to study the conformational properties of HVRs from M4 and M22 proteins in isolation and in complex with the M protein binding portion of C4BP. We conclude that the HVRs of M4 and M22 are folded as coiled coils and that the folded nucleus of the M4 HVR has a length of approximately 27 residues. Moreover, we demonstrate that the C4BP binding surface of M4-N is found within a region of four heptad repeats. Using molecular modeling, we propose a model for the structure of the M4 HVR that is consistent with our experimental information from NMR spectroscopy.
André
I
I
Department of Biophysical Chemistry, Lund University, Chemical Center, S-221 00 Lund, Sweden.
Persson
J
J
Blom
A M
AM
Nilsson
H
H
Drakenberg
T
T
Lindahl
G
G
Linse
S
S
D000595:Amino Acid Sequence; D000942:Antigens, Bacterial / Q000737:chemistry*; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry*; D002352:Carrier Proteins / Q000737:chemistry*; D002942:Circular Dichroism; D050716:Complement C4b-Binding Protein; D022801:Complementarity Determining Regions / Q000737:chemistry*; D006649:Histocompatibility Antigens / Q000737:chemistry*; D006801:Humans; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
D000942:Antigens, Bacterial; D001425:Bacterial Outer Membrane Proteins; C055609:C4BPA protein, human; D002352:Carrier Proteins; D050716:Complement C4b-Binding Protein; D022801:Complementarity Determining Regions; D006649:Histocompatibility Antigens; D011994:Recombinant Proteins; C014689:streptococcal M protein