With more than 80 members worldwide, the Orthobunyavirus genus in the Peribunyaviridae family is a large genus of enveloped RNA viruses, many of which are emerging pathogens in humans and livestock. How orthobunyaviruses (OBVs) penetrate and infect mammalian host cells remains poorly characterized. Here, we investigated the entry mechanisms of the OBV Germiston (GERV). Viral particles were visualized by cryo-electron microscopy and appeared roughly spherical with an average diameter of 98 nm. Labeling of the virus with fluorescent dyes did not adversely affect its infectivity and allowed the monitoring of single particles in fixed and live cells. Using this approach, we found that endocytic internalization of bound viruses was asynchronous and occurred within 30-40 min. The virus entered Rab5a+ early endosomes and, subsequently, late endosomal vacuoles containing Rab7a but not LAMP-1. Infectious entry did not require proteolytic cleavage, and endosomal acidification was sufficient and necessary for viral fusion. Acid-activated penetration began 15-25 min after initiation of virus internalization and relied on maturation of early endosomes to late endosomes. The optimal pH for viral membrane fusion was slightly below 6.0, and penetration was hampered when the potassium influx was abolished. Overall, our study provides real-time visualization of GERV entry into host cells and demonstrates the importance of late endosomal maturation in facilitating OBV penetration. IMPORTANCE Orthobunyaviruses (OBVs), which include La Crosse, Oropouche, and Schmallenberg viruses, represent a growing threat to humans and domestic animals worldwide. Ideally, preventing OBV spread requires approaches that target early stages of infection, i.e., virus entry. However, little is known about the molecular and cellular mechanisms by which OBVs enter and infect host cells. Here we developed accurate, sensitive tools and assays to investigate the penetration process of the OBV Germiston (GERV). Our data emphasize the central role of the late endosomal maturation in GERV entry, providing a comprehensive overview of the early stages of an OBV infection. Our study also brings a complete toolbox of innovative methods to study each step of the OBV entry program in fixed and living cells, from virus binding and endocytosis to fusion and penetration. The information gained herein lay the foundation for the development of antiviral strategies aiming to block OBV entry.

The sand fly-borne Toscana virus (TOSV) is the major cause of human meningoencephalitis in the Mediterranean basin during the summer season. In this work, we have developed a T7 RNA polymerase-driven reverse genetics system to recover infectious particles of a lineage B strain of TOSV. The viral protein pattern and growth properties of the rescued virus (rTOSV) were found to be similar to those of the corresponding wild-type (wt) virus. Using this system, we genetically engineered a TOSV mutant lacking expression of the non-structural protein NSs (rTOSV Viruses 2019, 11, x FOR PEER REVIEW and Proteomics Core Facility of the German Cancer Research Center, Heidelberg (Table 1). B viruses were produced and purified, as described in Section 2.5. Soluble ectodomains of GN a were stably expressed in drosophila S2 cells and purified according to a standard procedure Using these protocols, the protein purity reached over 90%. Antisera were prepared in guinea by injecting 100 µg of Triton X-100-inactivated purified viruses or soluble ectodomains of GN a in Freund's complete adjuvant. The priming was followed by three booster injections of 100 µg week intervals, the first one in Freund's incomplete adjuvant, and the others in phosphate-bu saline (PBS). Animals were bled before the first immunization (control pre-immune serum) days after the last injection. The purified rabbit polyclonal antibodies against the TOSV nucleop N (T4) and non-structural protein NSs (T5) were developed by GenScript (Piscataway Netherlands). Rabbits were immunized with either a peptide corresponding to the C termin amino acid residues of NSs or the full-length N protein with a C-terminal His-tag. The m immune ascitic fluid against all TOSV structural proteins was a generous gift from R.B. (University of Texas, Galveston, Texas, USA). s were constructed by GenScript. Briefly, the cDNAs encoding the full-length S, M, and f TOSV, flanked by an upstream T7 polymerase promoter sequence and a downstream ta virus (HdV) ribozyme sequence ( Figure S1), were synthesized. The gene synthesis e subjected to blunt-end ligation into EcoRV-linearized pUC57 plasmid, or alternatively, into the pCC1 plasmid, using the CloneEZ PCR Cloning Kit (Genscript), resulting in C1-M, and pUC57-L. In addition, a pUC57-SɸNSs was created in which the S segment mRNA, with the 18 first AUG codons replaced by UAG stop codons ( Figure S2). Viruses from Plasmid DNAscells expressing T7 polymerase were seeded in 6-well plates (2.5 × 10 5 cells per well). g day, rTOSV was rescued from cells by transfection with 1 µg each of the plasmids C1-M, and pUC57-L. Alternatively, the recovery of rTOSVɸNSs was achieved following thod but using the plasmid pUC57-SɸNSs instead of the pUC57-S. Transfection was ith Lipofectamine 2000 (Thermo Fisher Scientific), using a ratio of 1 µL Lipofectamine of plasmids in 400 µL of complete GMEM without antibiotics. Supernatants were fresh culture medium containing antibiotics and 2% serum 4 h post-transfection. Five nsfection, supernatants were harvested, c...

With over 80 members worldwide, Orthobunyavirus is the largest genus in the Peribunyaviridae family. Orthobunyaviruses (OBVs) are arthropod-borne viruses that are structurally simple, with a trisegmented, negative-sense RNA genome and only four structural proteins. OBVs are potential agents of emerging and re-emerging diseases and overall represent a global threat to both public and veterinary health. The focus of this review is on the very first steps of OBV infection in mammalian hosts, from virus binding to penetration and release of the viral genome into the cytosol. Here, we address the most current knowledge and advances regarding OBV receptors, endocytosis, and fusion.