Identification and Characterization of the Nucleolar Localization Signal of Autographa californica Multiple Nucleopolyhedrovirus LEF5

Abstract

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) late expression factor 5 (LEF5) is highly conserved in all sequenced baculovirus genomes and plays an important role in production of infectious viral progeny. In this study, nucleolar localization of AcMNPV LEF5 was characterized. Through transcriptome analysis, we identified two putative nucleolar proteins, Spodoptera frugiperda nucleostemin (SfNS) and fibrillarin (SfFBL), from Sf9 cells. Immunofluorescence analysis demonstrated that SfNS and SfFBL were localized to the nucleolus. AcMNPV infection resulted in reorganization of the nucleoli of infected cells. Colocalization of LEF5 and SfNS showed that AcMNPV LEF5 was localized to the nucleolus in Sf9 cells. Bioinformatic analysis revealed that basic amino acids of LEF5 are enriched at residues 184 to 213 and may contain a nucleolar localization signal (NoLS). Green fluorescent protein (GFP) fused to NoLS of AcMNPV LEF5 localized to the nucleoli of transfected cells. Multiple-point mutation analysis demonstrated that amino acid residues 197 to 204 are important for nucleolar localization of LEF5. To identify whether the NoLS in AcMNPV LEF5 is important for production of viral progeny, a lef5-null AcMNPV bacmid was constructed; several NoLS-mutated LEF5 proteins were reinserted into the lef5-null AcMNPV bacmid with a GFP reporter. The constructs containing point mutations at residues 185 to 189 or 197 to 204 in AcMNPV LEF5 resulted in reduction in production of infectious viral progeny and occlusion body yield in bacmid-transfected cells. Together, these data suggested that AcMNPV LEF5 contains an NoLS, which is important for nucleolar localization of LEF5, progeny production, and occlusion body production.IMPORTANCE Many viruses, including human and plant viruses, target nucleolar functions as part of their infection strategy. However, nucleolar localization for baculovirus proteins has not yet been characterized. In this study, two nucleolar proteins, SfNS and SfFBL, were identified in Sf9 cells. Our results showed that Autographa californica multiple nucleopolyhedrovirus (AcMNPV) infection resulted in redistribution of the nucleoli of infected cells. We demonstrated that AcMNPV late expression factor 5 (LEF5) could localize to the nucleolus and contains a nucleolar localization signal (NoLS), which is important for nucleolar localization of AcMNPV LEF5 and for production of viral progeny and yield of occlusion bodies.

Keywords: baculovirus; late expression factor 5; nucleolar localization; nucleolar protein.