The bacterial flagellar filament is a helical propeller constructed from 11 protofilaments of a single protein, flagellin. The filament switches between left- and right-handed supercoiled forms when bacteria switch their swimming mode between running and tumbling. Supercoiling is produced by two different packing interactions of flagellin called L and R. In switching from L to R, the intersubunit distance ( approximately 52 A) along the protofilament decreases by 0.8 A. Changes in the number of L and R protofilaments govern supercoiling of the filament. Here we report the 2.0 A resolution crystal structure of a Salmonella flagellin fragment of relative molecular mass 41,300. The crystal contains pairs of antiparallel straight protofilaments with the R-type repeat. By simulated extension of the protofilament model, we have identified possible switch regions responsible for the bi-stable mechanical switch that generates the 0.8 A difference in repeat distance.
The growth of the bacterial flagellar filament occurs at its distal end by self-assembly of flagellin transported from the cytoplasm through the narrow central channel. The cap at the growing end is essential for its growth, remaining stably attached while permitting the flagellin insertion. In order to understand the assembly mechanism, we used electron microscopy to study the structures of the cap-filament complex and isolated cap dimer. Five leg-like anchor domains of the pentameric cap flexibly adjusted their conformations to keep just one flagellin binding site open, indicating a cap rotation mechanism to promote the flagellin self-assembly. This represents one of the most dynamic movements in protein structures.
1. INTRODUCTION 22. OVERALL STRUCTURE AND SUBSTRUCTURES 52.1 Overall structure and components 52.2 Bidirectional rotary motor 52.3 Drive shaft 82.4 Bushing 82.5 Universal joint 92.6 Helical propeller 92.7 Axial junction 102.8 Capping structure 113. ASSEMBLY PROCESS OF THE FLAGELLUM 113.1 Step by step assembly 113.2 Flagellum-specific export apparatus and the channel 124. UNIQUE CHARACTERISTICS OF THE FLAGELLAR MOTOR DYNAMICS 135. STRUCTURAL DESIGN OF FLAGELLIN FOR ASSEMBLY REGULATION AND POLYMORPHISM 145.1 Domain organization and terminal disorder of flagellin 155.2 The role of terminal disorder in filament formation and polymorphism 175.3 Common structural motif for regulation of self-assembly 216. STRUCTURAL DESIGN OF FLAGELLAR FILAMENTS FOR POLYMORPHISM 226.1 Polymorphic mechanism 236.2 Structures of the filaments deduced by electron microscopy 256.2.1 Overview of the electron microscopic studies 256.2.2 Helical image reconstruction procedure 276.2.3 Structural details of the filament 286.3 X-ray fibre diffraction studies 326.3.1 Overview of the X-ray studies 326.3.2 Orientation of liquid crystalline sols and diffraction patterns 336.3.3 Equatorial analysis 356.3.4 A preliminary map refined at 11 Åresolution 376.4 Overall chain folding of the subunit in the filament 386.4.1 Mapping out the terminal and central regions 386.4.2 The chain folding and role of each domain 426.5 Polymorphic nature of flagellar filament 436.5.1 Comparison of the L- and R-type 436.5.2 New helical symmetry ‘Lt-type’ 466.5.3 Direct comparison of the Lt-type lattice to the other two 486.5.4 Plausible conformational changes involved in polymorphism 517. PERSPECTIVE 558. ACKNOWLEDGEMENTS 559. REFERENCES 55
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