Prokaryotic motility structures

Abstract

Prokaryotes use a wide variety of structures to facilitate motility. The majority of research to date has focused on swimming motility and the molecular architecture of the bacterial flagellum. While intriguing questions remain, especially concerning the specialized export system involved in flagellum assembly, for the most part the structural components and their location within the flagellum and function are now known. The same cannot be said of the other apparati including archaeal flagella, type IV pili, the junctional pore, ratchet structure and the contractile cytoskeleton used by a variety of organisms for motility. In these cases, many of the structural components have yet to be identified and the mechanism of action that results in motility is often still poorly understood. Research on the bacterial flagellum has greatly aided our understanding of not only motility but also protein secretion and genetic regulation systems. Continued study and understanding of all prokaryotic motility structures will provide a wealth of knowledge that is sure to extend beyond the bounds of prokaryotic movement. OverviewMotility is widespread throughout the prokaryotes, yet no one structure confers motility to all organisms in all circumstances. Of the motility structures, the bacterial flagellum has received the most attention from researchers. There exist a number of variations on the classical flagellum, such as different lateral and polar flagella on the same cell, and the periplasmic flagella of spirochaetes. Motility is also conferred by flagella in the domain Archaea, yet these structures bear little similarity to their bacterial counterparts. Rather, archaeal flagella demonstrate similarity to another bacterial motility apparatus, type IV pili. Additional structures involved in bacterial motility include the junctional pore complex and the ratchet structure involved in gliding motility, and the unique contractile cytoskeleton of Spiroplasma. Bacterial flagellaWithout a doubt the most common and best studied of all prokaryotic motility structures is the bacterial flagellum (Aldridge & Hughes, 2002;Macnab, 1999). Composed of over 20 protein species with approximately another 30 proteins required for regulation and assembly, it is one of the most complex of all prokaryotic organelles (Fig. 1). Well understood in its own right as a motility structure, it has become a model system for type III secretion systems in general (Aldridge & Hughes, 2002).The bacterial flagellum is a rotary structure driven from a motor at the base, with the filament acting as a propeller. The flagellum consists of three major substructures: the filament, the hook and the basal body. The filament is typically about 20 nm in diameter and usually consists of thousands of copies of a single protein called flagellin. Less commonly the filament is composed of several different flagellins. At the tip of the flagellum is the capping protein HAP2. Connecting the filament to the basal body is the hook region, composed of a single protein. The junction ...

Stepwise formation of the bacterial flagellar system

“…Whereas E. coli and Salmonella have long served as the model organisms for studying flagellar assembly (2), there is extensive diversity among bacteria in the contents and organization of the gene complexes that specify flagella as well as structural variation in the flagellum itself (8,9). For example, in Spirochaetes, flagella are located in the periplasm between the outer membrane sheath and cell cylinder (10); and, in accordance with their location, they have an enlarged C ring and rotor, and have a shape different from that seen in Salmonella (11).…”

Type IV pili: paradoxes in form and function

“…Several key components are utilized in all Type IV pilus systems and are also found in Type II secretion and archaeal flagellar systems [3][4][5]. These are: the pilin subunit; an inner membrane prepilin peptidase that cleaves the N-terminal leader peptide; an assembly *email: licraig@sfu.ca.…”

Antigen-43-mediated autoaggregation impairs motility in Escherichia coli

“…Flagella and type IV pili are prominent organelles that contribute to motility Abbreviations: Ag43, antigen 43; AT, autotransporter. (Bardy et al, 2003;Harshey, 2003). Co-ordinated expression of these organelles is achieved by tightly regulated signalling systems that involve the various genes encoding them (up to 50 genes contribute to E. coli flagella biosynthesis), which are, in some cases, coupled to quorum sensing systems (Harshey, 2003;Soutourina & Bertin, 2003).…”