Exogenous lipid utilization by the gram-negative Vibrio cholerae
To overcome the membrane permeability barrier and low nutrient availability in the environment, Gram-negative organisms have evolved many mechanisms dedicated to the intake of nutrients. One such mechanism is the long-chain fatty acid uptake pathway. This pathway involves machinery that transports fatty acids across the outer membrane and into the cell, where the lipid can be used for either nutrition or for remodeling the structure of the membrane. Interestingly, the fatty acid species that can be recognized by this machinery differ between organisms; the aquatic pathogen Vibrio cholerae demonstrates a much wider substrate recognition profile than other Gram-negative species. In this work we elaborate on the lipid nutrients accessible to V. cholerae, demonstrating that it can use lysophosphatidylcholine as both a carbon source as well as a source of fatty acids for remodeling its outer membrane. We identify the enzyme responsible for the breakdown of lysophosphatidylcholine, VolA (Vibrio outer membrane lysophospholipase A). VolA is conserved in many Vibrio species as well as other aquatic Gram-negatives, annotated as a putative lipase. We show VolA is co-expressed with the outer membrane fatty acid transporter FadL; FadL serves to transport across the outer membrane those long-chain fatty acids freed from lysophosphatidylcholine by VolA. VolA is expressed on the surface of the cell as a membrane anchored lipoprotein; this is novel as VolA is the first identified example of a surface-localized lipoprotein phospholipase. Biochemical characterization of VolA shows that it acts as a canonical lysophospholipase in vitro, suggesting that it works in tandem with the FadL transporter, freeing fatty acids from lysophosphatidylcholine at the surface of the cell to be brought in via the fatty acid uptake pathway. This work expands on the currently understood lipid uptake abilities of Vibrio cholerae, demonstrating a novel mechanism for utilizing a nutrient not previously thought to metabolized. VolA is an important to our understanding of the larger picture of lipid uptake and how it contributes to the survival of Gram-negative organisms.