Dual Functions of the Protein MgtE in Pseudomonas aeruginosa

Abstract

The Gram-negative bacterium Pseudomonas aeruginosa is an opportunistic pathogen which readily establishes itself in the lungs of people with cystic fibrosis (CF). Most CF patients have life-long P. aeruginosa infections. By modulating its own virulence and forming biofilms, P. aeruginosa is able to evade both host immune responses and antibiotic treatments. Previous studies have shown that the magnesium transporter MgtE plays a role in virulence modulation by inhibiting transcription of the type III secretion system, a mechanism by which bacteria inject toxins directly into the eukaryotic host cell. MgtE had already been identified as a magnesium transporter, and thus its role in regulating cytotoxicity was indicative of dual functions for this protein. This research focused on a structure-function analysis of MgtE, with the hypothesis that the magnesium transport and cytotoxicity functions could be exerted independently. Cytotoxicity assays were conducted using a co-culture model system of cystic fibrosis bronchial epithelial cells and a ∆mgtE strain of P. aeruginosa transformed with plasmids carrying wild type or mutated mgtE. Magnesium transport was assessed using the same mgtE plasmids in a Salmonella strain deficient in all magnesium transporters. Through analysis of a number of mgtE mutants, we found two constructs – a mutation in a putative magnesium binding site, and an N-terminal truncation – which demonstrated a separation of functions. We further demonstrated the uncoupling of functions by showing that different mgtE mutants vary widely in their ability to regulate cytotoxicity, whether or not they are able to transport magnesium. Overall, these results support the hypothesis of MgtE as a dual function protein and may lead to a better understanding of the mechanisms underlying P. aeruginosa virulence. By understanding virulence mechanisms, we may be able to develop treatments to reduce infections and pave the way to better health for people with cystic fibrosis.