ABSTRACT
Introduction Flagella are the organelles for bacterial locomotion. These supramolecular structures extend from
the cytoplasm to the cell exterior and are composed of three major structural elements, the basal body, the hook and the filament (Fig. 1 ).1,2 The swimming pattern of bacteria, such as Eschenchia coli and Salmonella, consists of straight swimming for a few seconds and tumbling for a fraction of a second. During the straight swimming phase, the helical filaments form a bundle behind the cell body, where the filaments are all in a left-handed supercoiled form, each acting as a propeller driven by a rotary motor at the base of the flagellum. Bacteria tumble once every few seconds to change their swimming direction for the chemotactic behavior. The tumbling is triggered by quick reversal of motor rotation, which produces a twist in the filament structure and transforms it into right-handed supercoils momentarily. This allows the bundle to fall apart smoothly and then the uncoordinated propelling forces change the orientation of the cell quickly.3,4 The flagellar filaments can also be transformed into various but distinct polymorphic forms including two straight forms, in response to amino acid replacements in flagellin5 and to chemical changes in the environment6,7 as well as to mechanical forces.3,8
The long helical filament is connected to the basal structure via a short, highly curved segment called the hook.9 Both the hook and the helical filament are self-assembling macromolecular
♦Corresponding Author: Ferenc Vonderviszt-Department of Nanotechnology, Faculty of Information Technology, University of Pannonia, Egyetem u. 10, H-8200 Veszprém, Hungary. Email: von007@almos.vein.hu______________________________________________________
structures composed o f the hook protein (FlgE) and flagellin (FliC), respectively. Each filament may comprise as many as —30000 flagellin subunits and can grow up to —15 μπι. The hook is a helical assembly o f-130 copies of FlgE subunits with a well regulated length of 55 nm ± 6 nm,10 capable of forming polymorphic supercoil structures.11 Bacterial flagellar hook acts as a molecular universal joint that can transmit the torque produced by the basal body, a rotary motor, to the flagellar filament.
