The T4bSS of Legionella features a two-step secretion pathway with an inner membrane intermediate for secretion of transmembrane effectors

To promote intracellular survival and infection, Legionella spp. translocate hundreds of effector proteins into eukaryotic host cells using a type IV b protein secretion system (T4bSS). T4bSS are well known to translocate soluble as well as transmembrane domain-containing effector proteins (TMD-effectors) but the mechanisms of secretion are still poorly understood. Herein we investigated the secretion of hydrophobic TMD-effectors, of which about 80 were previously reported to be encoded by L. pneumophila. A proteomic analysis of fractionated membranes revealed that TMD-effectors are targeted to and inserted into the bacterial inner membranes of L. pneumophila independent of the presence of a functional T4bSS. While the T4bSS chaperones IcmS and IcmW were critical for secretion of all tested TMD-effectors, they did not influence inner membrane targeting of these proteins. As for soluble effector proteins, translocation of TMD-effectors into host cells depended on a C-terminal secretion signal and this signal needed to be presented towards the cytoplasmic side of the inner membrane. A different secretion behavior of TMD- and soluble effectors and the need for small periplasmic loops within TMD-effectors provided strong evidence that TMD-effectors are secreted in a two-step secretion process: Initially, an inner membrane intermediate is formed, that is extracted towards the cytoplasmic side, possibly by the help of the type IV coupling protein complex and subsequently secreted into eukaryotic host cells by the T4bSS core complex. Overall, our study highlights the amazing versatility of T4bSS to secrete soluble and TMD-effectors from different subcellular locations of the bacterial cell.

YgfB increases β-lactam resistance in Pseudomonas aeruginosa by counteracting AlpA-mediated ampDh3 expression

YgfB-mediated β-lactam resistance was recently identified in multi drug resistant Pseudomonas aeruginosa. We show that YgfB upregulates expression of the β-lactamase AmpC by repressing the function of the regulator of the programmed cell death pathway AlpA. In response to DNA damage, the antiterminator AlpA induces expression of the alpBCDE autolysis genes and of the peptidoglycan amidase AmpDh3. YgfB interacts with AlpA and represses the ampDh3 expression. Thus, YgfB indirectly prevents AmpDh3 from reducing the levels of cell wall-derived 1,6-anhydro-N-acetylmuramyl-peptides, required to induce the transcriptional activator AmpR in promoting the ampC expression and β-lactam resistance. Ciprofloxacin-mediated DNA damage induces AlpA-dependent production of AmpDh3 as previously shown, which should reduce β-lactam resistance. YgfB, however, counteracts the β-lactam enhancing activity of ciprofloxacin by repressing ampDh3 expression and lowering the benefits of this drug combination. Altogether, YgfB represents an additional player in the complex regulatory network of AmpC regulation.