Membrane Retention of West Nile Virus NS5 Depends on NS1 or NS3 for Enzymatic Activity.

West Nile virus (WNV) nonstructural protein 5 (NS5) possesses multiple enzymatic domains essential for viral RNA replication. During infection, NS5 predominantly localizes to unique replication organelles (ROs) at the rough endoplasmic reticulum (RER), known as vesicle packets (VPs) and convoluted membranes (CMs), with a portion of NS5 accumulating in the nucleus. NS5 is a soluble protein that must be in the VP, where its enzymatic activities are required for viral RNA synthesis. However, the mechanistic processes behind the recruitment of NS5 from the cytoplasm to the RER membrane remain unclear. Here, we utilize high-resolution confocal microscopy and sucrose density gradient ultracentrifugation to investigate whether the association of NS5 with other NS proteins contributes to its membrane recruitment and retention. We demonstrate that NS1 or NS3 partially influences the NS5 association with the membrane. We further demonstrate that processed NS5 is predominantly in the cytoplasm and nucleus, indicating that the processing of NS5 from the viral polyprotein does not contribute to its membrane localization. These observations suggest that other host or viral factors, such as the enwrapment of NS5 by the RO, may also be necessary for the complete membrane retention of NS5. Therefore, studies on the inhibitors that disrupt the membrane localization of WNV NS5 are warranted for antiviral drug development.

Potential Dual Role of West Nile Virus NS2B in Orchestrating NS3 Enzymatic Activity in Viral Replication.

West Nile virus (WNV) nonstructural protein 3 (NS3) harbors the viral triphosphatase and helicase for viral RNA synthesis and, together with NS2B, constitutes the protease responsible for polyprotein processing. NS3 is a soluble protein, but it is localized to specialized compartments at the rough endoplasmic reticulum (RER), where its enzymatic functions are essential for virus replication. However, the mechanistic details behind the recruitment of NS3 from the cytoplasm to the RER have not yet been fully elucidated. In this study, we employed immunofluorescence and biochemical assays to demonstrate that NS3, when expressed individually and when cleaved from the viral polyprotein, is localized exclusively to the cytoplasm. Furthermore, NS3 appeared to be peripherally recruited to the RER and proteolytically active when NS2B was provided in trans. Thus, we provide evidence for a potential additional role for NS2B in not only serving as the cofactor for the NS3 protease, but also in recruiting NS3 from the cytoplasm to the RER for proper enzymatic activity. Results from our study suggest that targeting the interaction between NS2B and NS3 in disrupting the NS3 ER localization may be an attractive avenue for antiviral drug discovery.

A cilevirus infects ornamental hibiscus in Hawaii.

The complete nucleotide sequence of a virus infecting ornamental hibiscus (Hibiscus sp.) in Hawaii with symptoms of green ringspots on senescing leaves was determined from double-stranded RNA isolated from symptomatic tissue. Excluding polyadenylated regions at the 3' termini, the bipartite RNA genome was 8748 and 5019 nt in length for RNA1 and RNA2, respectively. The genome organization was typical of a cilevirus: RNA1 encoded a large replication-associated protein with methyltransferase, protease, helicase and RNA-dependent RNA polymerase domains as well as a 29-kDa protein of unknown function. RNA2 possessed five open reading frames that potentially encoded proteins with molecular masses of 15, 7, 62, 32, and 24 kDa. The 32-kDa protein is homologous to 3A movement proteins of RNA viruses; the other proteins are of unknown function. A proteome comparison revealed that this virus was 92 % identical to citrus leprosis virus cytoplasmic type 2 (CiLV-C2), a recently characterized cilevirus infecting citrus with leprosis-like symptoms in Colombia. The high sequence similarity suggests that the virus described in this study could be a strain of CiLV-C2, but since the new genus Cilevirus does not have species demarcation criteria established at present, the classification of this virus infecting hibiscus is open to interpretation. This study represents the first documented case of a cilevirus established in the United States and provides insight into the diversity within the genus Cilevirus.