Secretins form large (~150 Å), homo-oligomeric, gated pores in the outer membrane (OM) of bacteria. Each secretin complex comprises 12-16 copies of the secretin protein. Indicative of the quintessential nature of this family of proteins, secretins are ubiquitous among all didermic bacterial phyla and also found in some phage genomes. They form the OM component of the Type Four Pili (T4P), the Type Two Secretion System (T2SS), the Type Three Secretion System (T3SS/Injectisome) and are also required for the assembly and export of filamentous phage (Inoviridae).
The T2SS is unique in that it exports a range of soluble folded exoproteins directly from the periplasm, commonly hydrolytic enzymes used to degrade biopolymers (proteins, carbohydrates, lipids) for nutrient acquisition, but also far more nefarious virulence factors such as the Cholera toxin, ETEC Heat-labile enterotoxin, or Pseudomonas Exotoxin A. Previously referred to as the terminal component of the general secretion system, it is now apparent that the T2SS is a more specialised secretion machine which must selectively recruit pre-exoproteins from the densely packed periplasm.
Here we present our recent advances in characterising bacterial T2SS secretins. The distribution and classification of these proteins will be discussed and our recent structural insights into the function and assembly of these complex molecules will be provided.