View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by Elsevier - Publisher Connector

Biochimica et Biophysica Acta 1694 (2004) 207–217 http://www.elsevier.com/locate/bba Review Type III flagellar export and flagellar assembly

Robert M. MacnabF

Department of Molecular Biophysics and Biochemistry, Yale University, 0734, 266 Whitney Avenue, P.O. Box 208114, New Haven, CT 06520-8114, USA Received 24 October 2003; accepted 29 April 2004 Available online 18 May 2004

Abstract

Bacterial flagella, unlike eukaryotic flagella, are largely external to the cell and therefore many of their subunits have to be exported. Export is ATP-driven. In Salmonella, the bacterium on which this chapter largely focuses, the apparatus responsible for flagellar protein export consists of six membrane components, three soluble components and several substrate-specific chaperones. Other flagellated eubacteria have similar systems. The membrane components of the export apparatus are housed within the flagellar basal body and deliver their substrates into a channel or lumen in the nascent structure from which point they diffuse to the far end and assemble. Both on the basis of sequence similarities of several components and structural similarities, the flagellar protein export systems clearly belong to the type III superfamily, whose other members are responsible for secretion of virulence factors by many species of pathogenic bacteria. D 2004 Elsevier B.V. All rights reserved.

Keywords: Bacterial ; Type III protein export; FliI ATPase; Morphogenesis; Chaperone

1. Introduction flagellum, the two organelles are in fact quite distinct. The bacterial flagellum is powered by proton-motive force (with Bacteria employ a number of different systems for contributions from the electrical potential Dw and the transferring from their cytoplasm to external struc- chemical potential DpH) or in some species sodium-motive tures, to the external medium, or into host cells. Each type force. The electrochemical energy is converted into torque, of system has its own characteristics such as substrate resulting in rotation of the flagellar motor, which is attached recognition motifs, presence or absence of cleavable signal via the MS ring to the basal body rod, to the hook, and then peptides, physical architecture, and export/secretion mech- to an extremely long thin helical structure, the flagellar anism. Many of these systems are described in other filament (Fig. 1). Rotation of this helical filament converts chapters in this issue. This chapter discusses the process torque into thrust and propels the cell. The system contains a of flagellar protein export by a type III pathway. It uses switch that can generate either counterclockwise or clock- Salmonella as its principal example but the same, or at least wise rotation. The two directions have different consequen- very similar, processes occur in all flagellated bacterial ces for the cell, forward motion and random reorientation. species. The chapter concludes with a brief comparison of Chemical signals modulate the probabilities of the two the similarities and differences between type III flagellar rotational directions so as to produce the beneficial behavior protein export and type III secretion of virulence factors. called . For extensive reviews of motility and The latter topic is dealt with in depth in the chapter by He. chemotaxis, see Refs. [1–4]. A recent review on flagellar assembly [5] includes a more extensive treatment of flagel- 1.1. The flagellum as motor organelle lar structure, motility and chemotaxis than that presented above. The bacterial flagellum is an organelle for cell propul- sion. Although it carries the same name as the eukaryotic 1.2. The flagellar gene system

Throughout this chapter, explicit mention