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XC6422 is a conserved hypothetical protein from Xanthomonas campestris pathovar campestris (Xcc), a Gram-negative yellow-pigmented pathogenic bacterium that causes black rot, one of the major worldwide diseases of cruciferous crops. The protein consists of 220 amino acids and its structure has been determined to 1.6 Å resolution using the multi-wavelength anomalous dispersion (MAD) method. Although it has very low sequence identity to protein sequences in the PDB (less than 20%), the determined structure nevertheless shows that it belongs to the superfamily of serine α/β-hydrolases, with an active site that is fully accessible to solvent owing to the absence of a lid domain. Modelling studies with the serine esterase inhibitor E600 indicate that XC6422 adopts a conserved Ser-His-Asp catalytic triad common to this superfamily and has a preformed oxyanion hole for catalytic activation. These structural features suggest that XC6422 is most likely to be a hydrolase active on a soluble ester or a small lipid. An extra strand preceding the first β-strand in the canonical α/β-hydrolase fold leads to extensive subunit interactions between XC6422 monomers, which may explain why XC6422 crystals of good diffraction quality can grow to dimensions of up to 1.5 mm in a few days. [ABSTRACT FROM AUTHOR]

Xanthomonas campestris pv. campestris is a Gram-negative yellow-pigmented pathogenic bacterium that causes black rot, one of the major worldwide diseases of cruciferous crops. Its genome contains approximately 4500 genes, roughly one third of which have no known structure and/or function. However, some genes of unknown function are highly conserved among several different bacterial genuses. XC6422 is one such conserved hypothetical protein and has been overexpressed in Escherichia coli, purified and crystallized in a variety of forms using the hanging-drop vapour-diffusion method. Crystals grew to approximately 2 × 1.5 × 0.4 mm in size after one week and diffracted to at least 1.6 Å resolution. They belong to the monoclinic space group C2, with one molecule per asymmetric unit and unit-cell parameters a = 75.8, b = 79.3, c = 38.2 Å, β = 109.4°. Determination of this structure may provide insights into the protein's function. [ABSTRACT FROM AUTHOR]