Streptococcal M Protein: α -Helical Coiled-Coil Structure and Arrangement on the Cell Surface

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 78; no. 8; pp. 4689 - 4693
• Main Authors: Phillips, George N., Flicker, Paula F., Cohen, Carolyn, Manjula, Belur N., Fischetti, Vincent A.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01-08-1981; National Acad Sciences
• Subjects: Amino acids; Antibodies, Bacterial - biosynthesis; Antigens, Bacterial; Bacterial Outer Membrane Proteins; Bacterial Proteins - immunology; C.D; Carrier Proteins; cell surface; Cell Wall - ultrastructure; Cell walls; Circular Dichroism; conformational analysis; Detergents; Dichroism; electron microscopy; Gels; Hydrogen Bonding; Microscopy, Electron; Molecular structure; Molecular Weight; Molecules; Protein Conformation; protein M; secondary structure; Streptococcus; Streptococcus pyogenes - ultrastructure; tropomyosin; ultracentrifugation; X ray diffraction
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.78.8.4689

The conformation and molecular dimensions of purified type 6 streptococcal M proteins establish the close structural relationship of these molecules to tropomyosin. Ultracentrifuge studies reveal that the M molecules exist as stable dimers; circular dichroism spectra indicate that the molecules contain about 70% α helix; and fiber x-ray diffraction diagrams show the characteristic reflections of the α -helical pattern. Electron microscopic images of M protein shadowed with platinum reveal rodshaped molecules having the same width as tropomyosin. However, the lengths of the M molecules are about 30% shorter than lengths predicted by assuming a completely α -helical molecule. These findings indicate that the structure of the M6 protein is primarily α -helical coiled coil. Comparison of the lengths of the fibers on the surface of the streptococcus and the isolated M proteins suggests that each fiber on the cell wall consists of a single M-protein molecule ≈ 500 angstrom long. The structure determined for these fimbriae is the first α -helical coiled-coil conformation to be demonstrated for bacterial surface projections.