Abstract
The type 5 secretion system (T5SS) is one of the more widespread secretion systems in Gram-negative bacteria. Proteins secreted by the T5SS are functionally diverse (toxins, adhesins, enzymes) and include numerous virulence factors. Mechanistically, the T5SS has long been considered the simplest of secretion systems, due to the paucity of proteins required for its functioning. Still, despite more than two decades of study, the exact process by which T5SS substrates attain their final destination and correct conformation is not totally deciphered. Moreover, the recent addition of new sub-families to the T5SS raises additional questions about this secretion mechanism. Central to the understanding of type 5 secretion is the question of protein folding, which needs to be carefully controlled in each of the bacterial cell compartments these proteins cross. Here, the biogenesis of proteins secreted by the Type 5 secretion system is discussed, with a focus on the various factors preventing or promoting protein folding during biogenesis.
Highlights
In bacteria, protein secretion is essential for numerous processes
IcsA allow post-translational, SecB-dependent targeting to the inner membrane (T5aSS) [65,66,67,68], proteins secreted by the two-partner secretion system (T5bSS TpsA pro[45,47,48,49] the deletion of extended signal peptide region” (ESPR) in EspP leads to co-translational, SRP-dependteins) [69,70], and trimeric ATs (T5cSS) [71,72], have been shown to be glycosylated at ent export [45,49]
The type 5 secretion system (T5SS) proteins, initially thought to be able to drive their own secretion, require numerous accessory factors for proper travel from their site of synthesis to their final destination. While most of these accessory factors have a general role in the biogenesis of secreted proteins, others appear specific to the T5SS
Summary
Protein secretion is essential for numerous processes (nutrients acquisition, pathogenesis, adaptation to the environment, etc.). The last addition to the T5SS, the T5fSS sub-family, includes Hops (Helicobacter outer membrane proteins), which are predicted to form an 8-stranded β-barrel in the OM, needed for the translocation of 15–110 kDa α-helical passenger domains. The passenger domain is not connected to one of the barrel termini, but instead constitutes one of the extracellular loops of this β-barrel (Figure 1) [32] Despite this variety in structure and topology, it is currently assumed that all proteins belonging to the T5SS have a rather similar mode of biogenesis. Toxins 2021, 13, 341 barrel allows the translocation of the passenger domain to the cell surface, where it adopts its final, functional conformation During their journey from their site of synthesis to the external medium, the folding of the different domains of T5SS proteins need to be carefully and temporally controlled.
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