|Journal of Virology Structure and Assembly|
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.
Human cytomegalovirus (HCMV) secondary envelopment requires the viral tegument protein pUL71. The lack of pUL71 results in a complex ultrastructural phenotype with increased numbers of viral capsids undergoing envelopment at the cytoplasmic virus assembly complex. Here, we report a role of the pUL71 C terminus in secondary envelopment. Mutant viruses expressing C-terminally truncated pUL71 (TB71del327-361 and TB71del348-351) exhibited an impaired secondary envelopment in transmission electron microscopy (TEM) studies. Further mutational analyses of the C terminus revealed a tetralysine motif whose mutation (TB71mutK348-351A) resulted in an envelopment defect that was undistinguishable from the defect caused by truncation of the pUL71 C terminus. Interestingly, not all morphological alterations that define the ultrastructural phenotype of a TB71stop virus were found in cells infected with the C-terminally mutated viruses. This suggests that pUL71 provides additional functions that modulate HCMV morphogenesis and are harbored elsewhere in pUL71. This is also reflected by an intermediate growth defect of the C-terminally mutated viruses compared to the growth of the TB71stop virus. Electron tomography and three-dimensional visualization of different stages of secondary envelopment in TB71mutK348-351A-infected cells showed unambiguously the formation of a bud neck. Furthermore, we provide evidence for progressive tegument formation linked to advancing grades of capsid envelopment, suggesting that tegumentation and envelopment are intertwined processes. Altogether, we identified the importance of the pUL71 C terminus and, specifically, of a positively charged tetralysine motif for HCMV secondary envelopment.
IMPORTANCE Human cytomegalovirus (HCMV) is an important human pathogen that causes severe symptoms, especially in immunocompromised hosts. Furthermore, congenital HCMV infection is the leading viral cause of severe birth defects. Development of antiviral drugs to prevent the production of infectious virus progeny is challenging due to a complex and multistep virion morphogenesis. The mechanism of secondary envelopment is still not fully understood; nevertheless, it represents a potential target for antiviral drugs. Our identification of the role of a positively charged motif in the pUL71 C terminus for efficient HCMV secondary envelopment underlines the importance of pUL71 and, especially, its C terminus for this process. It furthermore shows how cell-associated spread and virion release depend on secondary envelopment. Ultrastructural analyses of different stages of envelopment contribute to a better understanding of the mechanisms underlying the process of secondary envelopment. This may bring us closer to the development of novel concepts to treat HCMV infections.