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% a helix; and fiber x-ray diffraction diagrams show the characteristic reflections of the a-helical pattern. Electron microscopic images ofM 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 a-helical molecule. These findings indicate that the structure of the M6 protein is primarily a-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 A long. The structure determined for these fimbriae is the first a-helical coiled-coil conformation to be demonstrated for bacterial surface projections.Streptococcal M protein is an antiphagocytic molecule that forms a "fuzzy coat" offimbriae on the surface ofgroup A streptococci (1, 2). A striking physicochemical resemblance has been noted between M protein and the regulatory muscle protein, tropomyosin (3). In a subsequent report (4), this similarity was confirmed by the discovery of a characteristic seven-residue periodicity in three different M proteins. Because this same periodicity is present in tropomyosin and accounts for its a-helical coiled-coil conformation, it was suggested that this conformation also characterizes the structure of M proteins (4). We now report a more detailed study of the structure and conformation of M protein that further establishes its close relationship to tropomyosin.M protein was separated from type 6 streptococci by three different methods: digestion with pepsin (Pep M6 preparation) (5, 6), use of nonionic detergent (Det M6 preparation) (7), or disruption of the cell walls with purified phage lysin (Lys M6 preparation) (8). In this paper we describe physicochemical and structural characterizations of these three different, purified preparations ofM protein from type 6 streptococci. The number of polypeptide chains in the molecule was established by ultracentrifugation, the nature of the secondary structure was examined by circular dichroism and x-ray diffraction, and molecular dimensions were determined by visualization of individual molecules with electron microscopy. These observations suggest an arrangement for the M protein molecules on the cell surface. MATERIALS AND METHODSGroup A streptococcal strain D471 (M type 6) was from the Rockefeller University collection. Detergent-extracted M6 protein was prepared and purified by methods as described (7). M6 protein extracted by pepsin at pH 5.8 was purified essentially by methods described for Pep M5 (6). Lysin-extracted M6 protein was isolated by solubilization of ...

Abstract. We have calculated three-dimensional maps from images of myosin subfragment-1 (S1)-decorated thin filaments and Sl-decorated actin filaments preserved in frozen solution. By averaging many data sets we obtained highly reproducible maps that can be interpreted simply to provide a model for the native structure of decorated filaments. From our results we have made the following conclusions. The bulk of the actin monomer is ~65 x 40 x 40/~ and is composed of two domains. In the filaments the monomers are strongly connected along the genetic helix with weaker connections following the long pitch helix. The long axis of the monomer lies roughly perpendicular to the filament axis. The myosin head (S1) approaches the actin filament tangentially and binds to a single actin, the major interaction being with the outermost domain of actin. In the map the longest chord of S1 is ~130 ,~. The region of S1 closest to actin is of high density, whereas the part furthest away is poorly defined and may be disordered. By comparing maps from decorated thin filaments with those from decorated actin, we demonstrate that tropomyosin is bound to the inner domain of actin just in front of the myosin binding site at a radius of ~40/~. A small change in the azimuthal position of tropomyosin, as has been suggested by others to occur during Ca2+-mediated regulation in vertebrate striated muscle, appears to be insufficient to eclipse totally the major site of interaction between actin and myosin.

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Abstract. Dictyostelium myosin has been examined under conditions that reveal intramolecular and inter-