New insights on the role of the gamma-herpesvirus uracil-DNA glycosylase leucine loop revealed by the structure of the Epstein-Barr virus enzyme in complex with an inhibitor protein.

Epstein-Barr virus (EBV) is a human gamma-herpesvirus. Within its 86 open reading frame containing genome, two enzymes avoiding uracil incorporation into DNA can be found: uracil triphosphate hydrolase and uracil-DNA glycosylase (UNG). The latter one excises uracil bases that are due to cytosine deamination or uracil misincorporation from double-stranded DNA substrates. The EBV enzyme belongs to family 1 UNGs. We solved the three-dimensional structure of EBV UNG in complex with the uracil-DNA glycosylase inhibitor protein (Ugi) from bacteriophage PBS-2 at a resolution of 2.3 A by X-ray crystallography. The structure of EBV UNG encoded by the BKRF3 reading frame shows the excellent global structural conservation within the solved examples of family 1 enzymes. Four out of the five catalytic motifs are completely conserved, whereas the fifth one, the leucine loop, carries a seven residue insertion. Despite this insertion, catalytic constants of EBV UNG are similar to those of other UNGs. Modelling of the EBV UNG-DNA complex shows that the longer leucine loop still contacts DNA and is likely to fulfil its role of DNA binding and deformation differently than the enzymes with previously solved structures. We could show that despite the evolutionary distance of EBV UNG from the natural host protein, bacteriophage Ugi binds with an inhibitory constant of 8 nM to UNG. This is due to an excellent specificity of Ugi for conserved elements of UNG, four of them corresponding to catalytic motifs and a fifth one corresponding to an important beta-turn structuring the catalytic site.

Extraction of proteins from formalin-fixed, paraffin-embedded tissue using the Qproteome extraction technique and preparation of tryptic peptides for liquid chromatography/mass spectrometry analysis.

This unit provides a robust, reliable, and easy-to-use kit-based method for extraction of intact, non-degraded proteins from formalin-fixed, paraffin-embedded (FFPE) tissue, and their subsequent use for analysis by liquid chromatography/mass spectrometry (LC/MS). After deparaffinization, proteins are extracted from unstained sections of FFPE rat liver tissue. After a simple cleanup step using organic extraction, the sample is transferred into a buffer optimized for trypsin digestion of the extracted proteins. Subsequently, LC/MS is used to identify the proteins that gave rise to the tryptic peptides. Comparing formalin-fixed and frozen tissues, good correlation is observed in the mass spectrometric pattern attributable to the tryptic peptides and number of identified proteins. Since FFPE tissues are generally available in clinical practice, this method can be used to analyze biomarkers in different pathological situations (e.g., healthy vs. diseased). The method can also be used for protein extraction from fresh-frozen tissue.

A bridge crosses the active-site canyon of the Epstein-Barr virus nuclease with DNase and RNase activities.

Epstein-Barr virus, a double-stranded DNA (dsDNA) virus, is a major human pathogen from the herpesvirus family. The nuclease is one of the lytic cycle proteins required for successful viral replication. In addition to the previously described endonuclease and exonuclease activities on single-stranded DNA and dsDNA substrates, we observed an RNase activity for Epstein-Barr virus nuclease in the presence of Mn(2+), giving a possible explanation for its role in host mRNA degradation. Its crystal structure shows a catalytic core of the D-(D/E)XK nuclease superfamily closely related to the exonuclease from bacteriophage lambda with a bridge across the active-site canyon. This bridge may reduce endonuclease activity, ensure processivity or play a role in strand separation of dsDNA substrates. As the DNA strand that is subject to cleavage is likely to make a sharp turn in front of the bridge, endonuclease activity on single-stranded DNA stretches appears to be possible, explaining the cleavage of circular substrates.

Structural genomics of the Epstein-Barr virus.

Epstein-Barr virus is a herpesvirus that causes infectious mononucleosis, carcinomas and immunoproliferative disease. Its genome encodes 86 proteins, which provided targets for a structural genomics project. After updating the annotation of the genome, 23 open reading frames were chosen for expression in Escherichia coli, initially selecting for those with known enzyme activity and then supplementing this set based on a series of predicted properties, in particular secondary structure. The major obstacle turned out to be poor expression and low solubility. Surprisingly, this could not be overcome by modifications of the constructs, changes of expression temperature or strain or renaturation. Of the eight soluble proteins, five were crystallized using robotic nanolitre-drop crystallization trials, which led to four solved structures. Although these results depended on individual treatment rather than standardized protocols, a high-throughput miniaturized crystallization screening protocol was a key component of success with these difficult proteins.