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.
Summary
The CRISPR-Cas system is a prokaryotic host defense system against
genetic elements. The Type III-B CRISPR-Cas system of the bacterium
Thermus thermophilus, the TtCmr complex, is composed of six
different protein subunits (Cmr1-6) and one crRNA with a stoichiometry of
Cmr112131445361:crRNA1.
The TtCmr complex co-purifies with crRNA species of 40 and 46 nt, originating
from a distinct subset of CRISPR loci and spacers. The TtCmr complex cleaves the
target RNA at multiple sites with 6 nt intervals via a 5’ ruler
mechanism. Electron microscopy revealed that the structure of TtCmr resembles a
‘sea worm’ and is composed of a Cmr2-3 heterodimer
‘tail’, a helical backbone of Cmr4 subunits capped by Cmr5
subunits, and a curled ‘head’ containing Cmr1 and Cmr6. Despite
having a backbone of only four Cmr4 subunits and being both longer and narrower,
the overall architecture of TtCmr resembles that of Type I Cascade
complexes.
The bacterial flagellar filament is a helical propeller rotated by the flagellar motor for bacterial locomotion. The filament is a supercoiled assembly of a single protein, flagellin, and is formed by 11 protofilaments. For bacterial taxis, the reversal of motor rotation switches the supercoil between left- and right-handed, both of which arise from combinations of two distinct conformations and packing interactions of the L-type and R-type protofilaments. Here we report an atomic model of the L-type straight filament by electron cryomicroscopy and helical image analysis. Comparison with the R-type structure shows interesting features: an orientation change of the outer core domains (D1) against the inner core domains (D0) showing almost invariant orientation and packing, a conformational switching within domain D1, and the conformational flexibility of domains D0 and D1 with their spoke-like connection for tight molecular packing.
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