Crystal structure of a hypothetical T2SS effector Lpg0189 from Legionella pneumophila reveals a novel protein fold

Lpg0189 is a type II secretion system-dependent extracellular protein with unknown function from Legionella pneumophila. Herein, we determined the crystal structure of Lpg0189 at 1.98 Å resolution by using single-wavelength anomalous diffraction (SAD). Lpg0189 folds into a novel chair-shaped architecture, with two sheets roughly perpendicular to each other. Bioinformatics analysis suggests Lpg0189 and its homologues are unique to Legionellales and evolved divergently. The interlinking structural and bioinformatics studies provide a better understanding of this hypothetical protein.

Identification of a potent inhibitor of type II secretion system from Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen infecting human population. The pathogen is becoming a serious health problem due to its ability to evade normal immune response of the host and multiple drug resistance to many antibiotics. The pathogen has 2 major virulence systems of which the type III secretion system (T3SS) is of major concern to humans. A third system, type 2 secretion system (T2SS), is common to bacteria and used to secrete exotoxin A (ExoA) responsible for human cell destruction. To help bypass the drug resistance, a strategy to block the T2SS based on a low similarity between human ATPases and the essential ATPases of the T3SS and T2SS of P. aeruginosa, was used. An in silico-optimized inhibitor of T3SS, made directly from the computer-optimized of previously published compounds and their combinatorial libraries, showed IC₅₀ = 1.3 ± 0.2 μM in the T2SS ExoA secretion blocking test. The compound was non-toxic to human lung epithelial cell line A549 and could block cellular destruction of those cells in a cell infection model at 200 μM for at least 24 h. The compound could be a lead candidate for the development of T2SS virulence blockers of Pseudomonas aeruginosa.

A dodecameric ring-like structure of the N0 domain of the type II secretin from enterotoxigenic Escherichia coli

In many bacteria, secretins from the type II secretion system (T2SS) function as outer membrane gated channels that enable passage of folded proteins from the periplasm into the extracellular milieu. Cryo-electron microscopy of the T2SS secretin GspD revealed previously the dodecameric cylindrical architecture of secretins, and crystal structures of periplasmic secretin domains showed a modular domain organization. However, no high-resolution experimental data has as yet been provided about how the entire T2SS secretin or its domains are organized in a cylindrical fashion. Here we present a crystal structure of the N0 domain of the T2SS secretin GspD from enterotoxigenic Escherichia coli containing a helix with 12 subunits per turn. The helix has an outer diameter of ∼125Å and a pitch of only 24Å which suggests a model of a cylindrical dodecameric N0 ring whose dimensions correspond with the cryo-electron microscopy map of Vibrio cholerae GspD. The N0 domain is known to interact with the HR domain of the inner membrane T2SS protein GspC. When the new N0 ring model is combined with the known N0·HR crystal structure, a dodecameric double-ring of twelve N0-HR heterodimers is obtained. In contrast, the previously observed compact N0-N1 GspD module is not compatible with the N0 ring. Interestingly, a N0-N1 T3SS homolog is compatible with forming a N0-N1 dodecameric ring, due to a different N0-vs-N1 orientation. This suggests that the dodecameric N0 ring is an important feature of T2SS secretins with periplasmic domains undergoing considerable motions during exoprotein translocation.

Assembly of the type II secretion system

The type II secretion system is utilized by many Gram-negative bacteria to export folded proteins to the surface and/or the extracellular environment of the cell. Although the function of the system is to move proteins from the periplasm to the outside of the cell, it is a large trans-envelope structure composed of more than a dozen different proteins present in multiple copies, including peripheral, integral inner membrane and integral outer membrane proteins plus a pseudopilus stretching between them. The establishment of this structure as an integral component of the entire envelope including the peptidoglycan layer between the two membranes requires assembly. Many of the participants and processes involved in this assembly have now been established, while other aspects remain to be discovered or more fully understood.