Abstract
Protein secretion systems are vital for prokaryotic life, as they enable bacteria to acquire nutrients, communicate with other species, defend against biological and chemical agents, and facilitate disease through the delivery of virulence factors. In this review, we will focus on the recently discovered type IX secretion system (T9SS), a complex translocon found only in some species of the Bacteroidetes phylum. T9SS plays two roles, depending on the lifestyle of the bacteria. It provides either a means of movement (called gliding motility) for peace-loving environmental bacteria or a weapon for pathogens. The best-studied members of these two groups are Flavobacterium johnsoniae, a commensal microorganism often found in water and soil, and Porphyromonas gingivalis, a human oral pathogen that is a major causative agent of periodontitis. In P. gingivalis and some other periodontopathogens, T9SS translocates proteins, especially virulence factors, across the outer membrane (OM). Proteins destined for secretion bear a conserved C-terminal domain (CTD) that directs the cargo to the OM translocon. At least 18 proteins are involved in this still enigmatic process, with some engaged in the post-translational modification of T9SS cargo proteins. Upon translocation across the OM, the CTD is removed by a protease with sortase-like activity and an anionic LPS is attached to the newly formed C-terminus. As a result, a cargo protein could be secreted into the extracellular milieu or covalently attached to the bacterial surface. T9SS is regulated by a two-component system; however, the precise environmental signal that triggers it has not been identified. Exploring unknown systems contributing to bacterial virulence is exciting, as it may eventually lead to new therapeutic strategies. During the past decade, the major components of T9SS were identified, as well as hints suggesting the possible mechanism of action. In addition, the list of characterized cargo proteins is constantly growing. The actual structure of the translocon, situated in the OM of bacteria, remains the least explored area; however, new technical approaches and increasing scientific attention have resulted in a growing body of data. Therefore, we present a compact up-to-date review of this topic.
Highlights
DISCOVERY OF T9SSSecretion of hemolysin A by E. coli, described four decades ago, was the first protein secretion system discovered in Gramnegative bacteria
Eight other protein secretion pathways have been characterized in these prokaryotes, which have a cell envelope consisting of the inner membrane (IM) and the outer membrane (OM) separated by the periplasm
The available data suggest that the KLIKK proteases are secreted into the extracellular milieu without removal of the C-terminal domain (CTD). This finding is similar to the secretion of PorU and PorZ from P. gingivalis, where the CTD is not removed during secretion, proteins stay associated with the cell surface (Lasica et al, 2016)
Summary
DISCOVERY OF T9SSSecretion of hemolysin A by E. coli, described four decades ago, was the first protein secretion system discovered in Gramnegative bacteria (diderm bacteria; Goebel and Hedgpeth, 1982). PorU localizes to the surface of P. gingivalis cells and is involved in T9SS cargo processing (see the section; Glew et al, 2012; Gorasia et al, 2015). The protein without the C-terminal Ig-like domain of 72 amino acid residues was not secreted, but accumulated in the periplasm of the mutated P. gingivalis strain in its truncated form (Seers et al, 2006).
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