The bacterial flagellar filament is a helical propeller for bacterial locomotion. It is a helical assembly of a single protein, flagellin, and its tubular structure is formed by 11 protofilaments in two distinct conformations, L- and R-type, for supercoiling. The X-ray crystal structure of a flagellin fragment lacking about 100 terminal residues revealed the protofilament structure, but the full filament structure is still essential for understanding the mechanism of supercoiling and polymerization. Here we report a complete atomic model of the R-type filament by electron cryomicroscopy. A density map obtained from image data up to 4 A resolution shows the feature of alpha-helical backbone and some large side chains. The atomic model built on the map reveals intricate molecular packing and an alpha-helical coiled coil formed by the terminal chains in the inner core of the filament, with its intersubunit hydrophobic interactions having an important role in stabilizing the filament.
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