First Crystal Structure of a Nonstructural Hepatitis E Viral Protein Identifies a Putative Novel Zinc-Binding Protein

Hepatitis E virus (HEV) is a 7.2-kb positive-sense, single-stranded RNA virus containing three partially overlapping reading frames, ORF1 to ORF3. All nonstructural proteins required for viral replication are encoded by ORF1 and are transcribed as a single transcript. Computational analysis of the complete ORF1 polyprotein identified a previously uncharacterized region of predicted secondary structure bordered by two disordered regions coinciding partially with a region predicted as a putative cysteine protease. Following successful cloning, expression, and purification of this region, the crystal structure of the identified protein was determined and identified to have considerable structural homology to a fatty acid binding domain. Further analysis of the structure revealed a metal binding site, shown unambiguously to specifically bind zinc via a nonclassical, potentially catalytic zinc-binding motif. Based on the structural homology of the HEV protein with known structures, along with the presence of a catalytic zinc-binding motif, it is possible that the identified protein corresponds to the HEV protease, which could require activation or repression through the binding of a fatty acid. This represents a significant step forward in the characterization and the understanding of the molecular mechanisms of the HEV genome. We present analysis for the first time of this identified nonstructural protein, expanding the knowledge and understanding of the complex mechanisms of HEV biology.IMPORTANCE Hepatitis E virus (HEV) is an emerging virus found predominately in developing countries; it causes an estimated 20 million infections, which result in approximately 57,000 deaths a year. Although it is known that the nonstructural proteins of HEV ORF1 are expressed as a single transcript, there is debate as to whether ORF1 functions as a single polyprotein or if it is processed into separate domains via a viral or endogenous cellular protease. Here we present the first structural and biophysical characterization of an HEV nonstructural protein using a construct that has partially overlapping boundaries with the predicted putative cysteine protease.