Crystallization of a newly discovered histidine acid phosphatase from Francisella tularensis

Synchrotron-based macromolecular crystallography module for an undergraduate biochemistry laboratory course

This paper describes the introduction of synchrotron-based macromolecular crystallography (MX) into an undergraduate laboratory class. An introductory 2 week experimental module on MX, consisting of four laboratory sessions and two classroom lectures, was incorporated into a senior-level biochemistry class focused on a survey of biochemical techniques, including the experimental characterization of proteins. Students purified recombinant protein samples, set up crystallization plates and flash-cooled crystals for shipping to a synchrotron. Students then collected X-ray diffraction data sets from their crystals via the remote interface of the Molecular Biology Consortium beamline (4.2.2) at the Advanced Light Source in Berkeley, CA, USA. Processed diffraction data sets were transferred back to the laboratory and used in conjunction with partial protein models provided to the students for refinement and model building. The laboratory component was supplemented by up to 2 h of lectures by faculty with expertise in MX. This module can be easily adapted for implementation into other similar undergraduate classes, assuming the availability of local crystallographic expertise and access to remote data collection at a synchrotron source.

Proline: Mother Nature's cryoprotectant applied to protein crystallography

L-Proline is one of Mother Nature's cryoprotectants. Plants and yeast accumulate proline under freeze-induced stress and the use of proline in the cryopreservation of biological samples is well established. Here, it is shown that L-proline is also a useful cryoprotectant for protein crystallography. Proline was used to prepare crystals of lysozyme, xylose isomerase, histidine acid phosphatase and 1-pyrroline-5-carboxylate dehydrogenase for low-temperature data collection. The crystallization solutions in these test cases included the commonly used precipitants ammonium sulfate, sodium chloride and polyethylene glycol and spanned the pH range 4.6-8.5. Thus, proline is compatible with typical protein-crystallization formulations. The proline concentration needed for cryoprotection of these crystals is in the range 2.0-3.0 M. Complete data sets were collected from the proline-protected crystals. Proline performed as well as traditional cryoprotectants based on the diffraction resolution and data-quality statistics. The structures were refined to assess the binding of proline to these proteins. As observed with traditional cryoprotectants such as glycerol and ethylene glycol, the electron-density maps clearly showed the presence of proline molecules bound to the protein. In two cases, histidine acid phosphatase and 1-pyrroline-5-carboxylate dehydrogenase, proline binds in the active site. It is concluded that L-proline is an effective cryoprotectant for protein crystallography.

Francisella acid phosphatases inactivate the NADPH oxidase in human phagocytes

Francisella tularensis contains four putative acid phosphatases that are conserved in Francisella novicida. An F. novicida quadruple mutant (AcpA, AcpB, AcpC, and Hap [DeltaABCH]) is unable to escape the phagosome or survive in macrophages and is attenuated in the mouse model. We explored whether reduced survival of the DeltaABCH mutant within phagocytes is related to the oxidative response by human neutrophils and macrophages. F. novicida and F. tularensis subspecies failed to stimulate reactive oxygen species production in the phagocytes, whereas the F. novicida DeltaABCH strain stimulated a significant level of reactive oxygen species. The DeltaABCH mutant, but not the wild-type strain, strongly colocalized with p47(phox) and replicated in phagocytes only in the presence of an NADPH oxidase inhibitor or within macrophages isolated from p47(phox) knockout mice. Finally, purified AcpA strongly dephosphorylated p47(phox) and p40(phox), but not p67(phox), in vitro. Thus, Francisella acid phosphatases play a major role in intramacrophage survival and virulence by regulating the generation of the oxidative burst in human phagocytes.

Acid phosphatases do not contribute to the pathogenesis of type A Francisella tularensis

The intracellular pathogen Francisella tularensis is the causative agent of tularemia, a zoonosis that can affect humans with potentially lethal consequences. Essential to Francisella virulence is its ability to survive and proliferate within phagocytes through phagosomal escape and cytosolic replication. Francisella spp. encode a variety of acid phosphatases, whose roles in phagosomal escape and virulence have been documented yet remain controversial. Here we have examined in the highly virulent (type A) F. tularensis strain Schu S4 the pathogenic roles of three distinct acid phosphatases, AcpA, AcpB, and AcpC, that are most conserved between Francisella subspecies. Neither the deletion of acpA nor the combination of acpA, acpB, and acpC deletions affected the phagosomal escape or cytosolic growth of Schu S4 in murine and human macrophages, despite decreases in acid phosphatase activities by as much as 95%. Furthermore, none of these mutants were affected in their ability to cause lethality in mice upon intranasal inoculation. Hence, the acid phosphatases AcpA, AcpB, and AcpC do not contribute to intracellular pathogenesis and do not play a major role in the virulence of type A Francisella strains.

Crystal Structures of the histidine acid phosphatase from Francisella tularensis provide insight into substrate recognition

Histidine acid phosphatases catalyze the transfer of a phosphoryl group from phosphomonoesters to water at acidic pH using an active-site histidine. The histidine acid phosphatase from the category A pathogen Francisella tularensis (FtHAP) has been implicated in intramacrophage survival and virulence, motivating interest in understanding the structure and mechanism of this enzyme. Here, we report a structure-based study of ligand recognition by FtHAP. The 1.70-A-resolution structure of FtHAP complexed with the competitive inhibitor l(+)-tartrate was solved using single-wavelength anomalous diffraction phasing. Structures of the ligand-free enzyme and the complex with inorganic phosphate were determined at resolutions of 1.85 and 1.70 A, respectively. The structure of the Asp261Ala mutant enzyme complexed with the substrate 3'-AMP was determined at 1.50 A resolution to gain insight into substrate recognition. FtHAP exhibits a two-domain fold similar to that of human prostatic acid phosphatase, consisting of an alpha/beta core domain and a smaller domain that caps the core domain. The structures show that the core domain supplies the phosphoryl binding site, catalytic histidine (His17), and an aspartic acid residue (Asp261) that protonates the leaving group, while the cap domain contributes residues that enforce substrate preference. FtHAP and human prostatic acid phosphatase differ in the orientation of the crucial first helix of the cap domain, implying differences in the substrate preferences of the two enzymes. 3'-AMP binds in one end of a 15-A-long tunnel, with the adenine clamped between Phe23 and Tyr135, and the ribose 2'-hydroxyl interacting with Gln132. The importance of the clamp is confirmed with site-directed mutagenesis; mutation of Phe23 and Tyr135 individually to Ala increases K(m) by factors of 7 and 10, respectively. The structural data are consistent with a role for FtHAP in scavenging phosphate from small molecules present in host macrophage cells.

Cloning, purification and crystallization of Bacillus anthracis class C acid phosphatase

Cloning, expression, purification and crystallization studies of a recombinant class C acid phosphatase from the Category A pathogen Bacillus anthracis are reported. Large diffraction-quality crystals were grown in the presence of HEPES and Jeffamine ED-2001 at pH 7.0. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 53.4, b = 90.1, c = 104.2 angstroms. The asymmetric unit is predicted to contain two protein molecules with a solvent content of 38%. Two native data sets were collected from the same crystal before and after flash-annealing. The first data set had a mosaicity of 1.6 degrees and a high-resolution limit of 1.8 angstroms. After flash-annealing, the apparent mosaicity decreased to 0.9 degrees and the high-resolution limit of usable data increased to 1.6 angstroms. This crystal form is currently being used to determine the structure of B. anthracis class C acid phosphatase with experimental phasing techniques.