Conformational change of flagellin for polymorphic supercoiling of the flagellar filament

Bacterial flagellar protofilaments can adopt 2 distinct conformations (L- or R-type), resulting in different functional states (the bacteria swim or tumble). The R-type protofilament was characterized previously by cryo-EM; now the same analysis of the L-type conformation provides insight into the conformational changes involved in this switch. 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.