Import pathways of the mannityl-opines into the bacterial pathogen Agrobacterium tumefaciens: structural, affinity and in vivo approaches.

Agrobacterium tumefaciens pathogens use specific compounds denoted opines as nutrients in their plant tumor niche. These opines are produced by the host plant cells genetically modified by agrobacteria. They are imported into bacteria via solute-binding proteins (SBPs) in association with ATP-binding cassette transporters. The mannityl-opine family encompasses mannopine, mannopinic acid, agropine and agropinic acid. Structural and affinity data on mannopinic acid bound to SBPs are currently lacking while those of the three others mannityl opines are available. We investigated the molecular basis of two pathways for mannopinic acid uptake. MoaA was proposed as the specific SBP for mannopinic acid import in mannityl opines-assimilating agrobacteria, which was validated here using genetic studies and affinity measurements. We structurally characterized the mannopinic acid-binding mode of MoaA in two crystal forms at 2.05 and 1.57 Šresolution. We demonstrated that the non-specific SBP MotA, so far characterized as mannopine and Amadori compound importer, was also able to transport mannopinic acid. The structure of MotA bound to mannopinic acid at 2.2 Šresolution defines a different mannopinic acid-binding signature, similar to that of mannopine. Combining in vitro and in vivo approaches, this work allowed us to complete the characterization of the mannityl-opines assimilation pathways, highlighting the important role of two dual imports of agropinic and mannopinic acids. Our data shed new light on how the mannityl-opines contribute to the establishment of the ecological niche of agrobacteria from the early to the late stages of tumor development.

A unique ferrous iron binding mode is associated with large conformational changes for the transport protein FpvC of Pseudomonas aeruginosa.

Pseudomonas aeruginosa secretes pyoverdine, a major siderophore to get access to iron, an essential nutrient. Pyoverdine scavenges ferric iron in the bacterial environment with the resulting complex internalized by bacteria. Releasing of iron from pyoverdine in the periplasm involves an iron reduction by an inner membrane reductase and two solute-binding proteins (SBPs) FpvC and FpvF in association with their ABC transporter. FpvC and FpvF belong to two different subgroups of SBPs within the structural cluster A: FpvC and FpvF were proposed to be a metal-binding protein and a ferrisiderophore-binding protein respectively. Here, we report the redox state and the binding mode of iron to FpvC. We first solved the crystal structure of FpvC bound to a fortuitous Ni2+ by single anomalous dispersion method. Using a different protein purification strategy, we determined the structure of FpvC with manganese and iron, which binds to FpvC in a ferrous state as demonstrated by electron paramagnetic resonance. FpvC is the first example of a hexahistidine metal site among SBPs in which the Fe2+ redox state is stabilized under aerobic conditions. Using biophysics methods, we showed that FpvC reversibly bind to a broad range of divalent ions. The structure of a mutant mimicking the apo FpvC reveals a protein in an open state with large conformational changes when compared with the metal-bound FpvC. These results highlight that the canonical metal site in FpvC is distinct from those yet described in SBPs and they provide new insights into the mechanism of PVD-Fe dissociation in P. aeruginosa.

Structural basis for two efficient modes of agropinic acid opine import into the bacterial pathogen Agrobacterium tumefaciens.

Agrobacterium tumefaciens pathogens genetically modify their host plants to drive the synthesis of opines in plant tumors. The mannityl-opine family encompasses mannopine, mannopinic acid, agropine and agropinic acid. These opines serve as nutrients and are imported into bacteria via periplasmic-binding proteins (PBPs) in association with ABC transporters. Structural and affinity data on agropine and agropinic acid opines bound to PBPs are currently lacking. Here, we investigated the molecular basis of AgtB and AgaA, proposed as the specific PBP for agropine and agropinic acid import, respectively. Using genetic approaches and affinity measurements, we identified AgtB and its transporter as responsible for agropine uptake in agropine-assimilating agrobacteria. Nonetheless, we showed that AgtB binds agropinic acid with a higher affinity than agropine, and we structurally characterized the agropinic acid-binding mode through three crystal structures at 1.4, 1.74 and 1.9 Šresolution. In the crystallization time course, obtaining a crystal structure of AgtB with agropine was unsuccessful due to the spontaneous lactamization of agropine into agropinic acid. AgaA binds agropinic acid only with a similar affinity in nanomolar range as AgtB. The structure of AgaA bound to agropinic acid at 1.65 Šresolution defines a different agropinic acid-binding signature. Our work highlights the structural and functional characteristics of two efficient agropinic acid assimilation pathways, of which one is also involved in agropine assimilation.

Structural Basis for High Specificity of Amadori Compound and Mannopine Opine Binding in Bacterial Pathogens.

Agrobacterium tumefaciens pathogens genetically modify their host plants to drive the synthesis of opines in plant tumors. Opines are either sugar phosphodiesters or the products of condensed amino acids with ketoacids or sugars. They are Agrobacterium nutrients and imported into the bacterial cell via periplasmic-binding proteins (PBPs) and ABC-transporters. Mannopine, an opine from the mannityl-opine family, is synthesized from an intermediate named deoxy-fructosyl-glutamine (DFG), which is also an opine and abundant Amadori compound (a name used for any derivative of aminodeoxysugars) present in decaying plant materials. The PBP MotA is responsible for mannopine import in mannopine-assimilating agrobacteria. In the nopaline-opine type agrobacteria strain, SocA protein was proposed as a putative mannopine binding PBP, and AttC protein was annotated as a mannopine binding-like PBP. Structural data on mannityl-opine-PBP complexes is currently lacking. By combining affinity data with analysis of seven x-ray structures at high resolution, we investigated the molecular basis of MotA, SocA, and AttC interactions with mannopine and its DFG precursor. Our work demonstrates that AttC is not a mannopine-binding protein and reveals a specific binding pocket for DFG in SocA with an affinity in nanomolar range. Hence, mannopine would not be imported into nopaline-type agrobacteria strains. In contrast, MotA binds both mannopine and DFG. We thus defined one mannopine and two DFG binding signatures. Unlike mannopine-PBPs, selective DFG-PBPs are present in a wide diversity of bacteria, including Actinobacteria, α-,ß-, and γ-proteobacteria, revealing a common role of this Amadori compound in pathogenic, symbiotic, and opportunistic bacteria.