As we have seen, there is a deep link between secretory systems and bacterial motility, and there is a strong link between the eubacterial flagellum and the type-III secretory system. Bacteria have multiple secretory systems, and we have already encountered the type-II system. Type-III secretory systems are involved primarily in secreting proteins that allow bacteria to attack and invade eu-karyotic cells.
The type-III secretory system forms a rivet structure identical to the rod and SMC-ring complex of the flagellum (Hueck 1998, Berry and Armitage 1999, Macnab 1999; see figure 6.1). Proteins exported by this system are shunted through the hollow SMC ring and through the rod to the outside of the cell (Hueck 1998, Berry and Armitage 1999, Macnab 1999). In flagellum assembly, flagellins and hook proteins are shunted to the outside of the cell via the rod-and-ring complex. The proteins attach to the outer rim of the rod and self-assemble into a tubular structure that will become the hook and filament, and flagellar proteins pass through this tube as it grows (Hueck 1998, Macnab 1999). Thus, the flagellum and the type-III secretory system share the same structure and function. This is no mere resemblance. Homology stud ies show that many of the flagellar proteins are related to parts of the type-III protein-secretion system (Hueck 1998, Berry and Armitage 1999, Macnab 1999), and the majority of the homology is in the rivet structure and the secretory apparatus (see figure 6.1).
Furthermore, the genes for the rivet rod and the SMC-ring complex form a single transcription unit in both the type-III secretory systems and the fla-gellum. The orientation and order of these genes in the transcription unit are very similar between the type-III secretory systems and the flagellum (Hueck 1998, Berry and Armitage 1999, Macnab 1999). Finally, phylogenetic studies suggest that type-III systems share a common ancestor (Aizawa 2001).
Importantly, the switching-torque generation system of the flagellum has homologs in virtually every type-III secretory system examined so far (Hueck 1998, Berry and Armitage 1999, Macnab 1999). Intriguingly, several type-III secretory systems have tubular structures attached to the rod. E. coli has a filamentous structure attached to one of its type-III secretory systems, which has significant similarity to the flagellar filament (Sekiya et al. 2001). While secretion in the flagellum is closely linked to flagellar assembly, it also plays a wider role. For example, pathogenic E. coli use the flagellar system to secrete enzymes that attack cell walls (Young et al. 1999).
While there is no apparent homolog of the motor (MotAB) in type-III secretory systems, the motor is homologous to the motor of the Tol-Pal and Exb-TonB secretory systems (Cascales et al. 2001). This homology links MotAB and the flagellum to a wide range of secretory systems, including the carbohydrate-secretory systems used in gliding motility (Youderian 2003). Major components of the gliding-secretory systems of Myxococcus xanthus are also related to the motor components of the Tol-Pal secretory system (Youderian 2003). Like the secretory and gliding-secretory systems (where deletion of the motor proteins stops secretion), deletion of MotAB not only paralyzes the fla-gellum but also significantly reduces secretion through the flagellum, emphasizing the dual role of the system (Young et al. 1999).
While the type-III secretory system does not have a chemical-sensing system like the eubacterial flagellum, close homologs of this system are present in the type-IV twitching-motility system and gliding-motility systems (Spormann 1999, Thomas et al. 2001).
Thus, there are deep links between the structure of the eubacterial fla-gellum and secretory systems. Flagella share the same basic structure as secretory systems, they secrete proteins as do secretory systems, motors that power secretory systems power them, and they are regulated by chemical-sensing systems that regulate other secretory systems. Indeed, Macnab (1999) considers flagella to be specialized type-III secretory systems.
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