Molecular structure of the Brucella abortus metalloprotein RicA, a Rab2-binding virulence effector.
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ABSTRACT: The Gram-negative intracellular pathogen Brucella abortus is the causative agent of brucellosis, which is among the most common zoonoses globally. The B. abortus RicA protein binds the host-expressed guanosine nucleotide-binding protein, Rab2, and modulates B. abortus infection biology. We have solved the first X-ray crystal structure of RicA to 2.7 Å resolution and have quantified the affinity of RicA binding to human Rab2 in its GDP-bound and nucleotide-free forms. RicA adopts a classic ?-carbonic anhydrase (?-CA) fold containing a left-handed ?-helix followed by a C-terminal ?-helix. Two homotrimers of RicA occupy the crystallographic asymmetric unit. Though no zinc was included in the purification or crystallization buffers, zinc is contained within the RicA crystals, as demonstrated by X-ray fluorescence spectroscopy. Electron density for a Zn(2+) ion coordinated by three histidine residues is evident in the putative active site of RicA. However, purified RicA preparations do not exhibit carbonic anhydrase activity, suggesting that Zn(2+) may not be the physiologically relevant metal cofactor or that RicA is not a bona fide carbonic anhydrase enzyme. Isothermal titration calorimetry (ITC) measurements of purified RicA binding to purified human Rab2 and GDP-Rab2 revealed similar equilibrium affinities (Kd ? 35 and 40 ?M, respectively). This study thus defines RicA as a Zn(2+)-binding ?-carbonic anhydrase-like protein that binds the human membrane fusion/trafficking protein Rab2 with low micromolar affinity in vitro. These results support a model in which ?-CA family proteins may evolve unique cellular functions while retaining many of the structural hallmarks of archetypal ?-CA enzymes.
SUBMITTER: Herrou J
PROVIDER: S-EPMC3902126 | biostudies-literature | 2013 Dec
REPOSITORIES: biostudies-literature
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