viruses Article The Change P82L in the Rift Valley Fever Virus NSs Protein Confers Attenuation in Mice Belén Borrego 1 , Sandra Moreno 1 , Nuria de la Losa 1 , Friedemann Weber 2 and Alejandro Brun 1, *   Citation: Borrego, B.; Moreno, S.; de la Losa, N.; Weber, F.; Brun, A.The Change P82L in the Rift Valley Fever Virus NSs Protein Confers Attenuation in Mice. Viruses 2021, 13, 542. https://doi.org/10.3390/v13040542 Academic Editors: Esther Schnettler and Benjamin Brennan Received: 21 January 2021 Accepted: 22 March 2021 Published: 24 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Centro de Investigación en Sanidad Animal, INIA-CISA, 28130 Valdeolmos, Spain; borrego@inia.es (B.B.); moreno.sandra@inia.es (S.M.); dllosa@inia.es (N.d.l.L.) 2 Institut für Virologie, Justus-Liebig-Universität, D-35392 Giessen, Germany; friedemann.weber@vetmed.uni-giessen.de * Correspondence: brun@inia.es Abstract: Rift Valley fever virus (RVFV) is a mosquito-borne bunyavirus that causes an important disease in ruminants, with great economic losses. The infection can be also transmitted to humans; therefore, it is considered a major threat to both human and animal health. In a previous work, we described a novel RVFV variant selected in cell culture in the presence of the antiviral agent favipiravir that was highly attenuated in vivo. This variant displayed 24 amino acid substitutions in different viral proteins when compared to its parental viral strain, two of them located in the NSs protein that is known to be the major virulence factor of RVFV. By means of a reverse genetics system, in this work we have analyzed the effect that one of these substitutions, P82L, has in viral attenuation in vivo. Rescued viruses carrying this single amino acid change were clearly attenuated in BALB/c mice while their growth in an interferon (IFN)-competent cell line as well as the production of interferon beta (IFN-β) did not seem to be affected. However, the pattern of nuclear NSs accumulation was modified in cells infected with the mutant viruses. These results highlight the key role of the NSs protein in the modulation of viral infectivity. Keywords: Rift Valley fever virus; non-structural NSs protein; interferon antagonist; nuclear filaments; PXXP motifs 1. Introduction Rift valley fever virus (RVFV) is a mosquito-borne phlebovirus of the Phenuiviridae family (O. Bunyavirales) that causes an important disease in ruminants, mostly characterized by a high-rate of abortions, fetal malformation and death of newborn lambs, with great economic losses. The infection can be transmitted to humans through mosquito bites or when exposed to infected material, producing a usually self-limiting disease with more severe development in a low percentage of cases (reviewed in [1]). Rift Valley fever is confined to the African continent and southern parts of the Arabian Peninsula and Indian Ocean islands, but its potential for spreading to other geographical areas linked to climatic change and globalization has been widely remarked [2]. Veterinary vaccines are available in Africa, but currently there are no licensed vaccines for human use, while in Europe there is no available treatment or licensed RVF vaccine. Therefore, the development of safer and effective control strategies intended also for human use is an active field of research [3–5]. The RVFV genome consists of three ssRNA(-) segments of different sizes (large, medium, small). The L-segment codes for an RNA-dependent RNA polymerase (RdRp). The M segment contains five in-frame start codons alternatively used by virtue of a riboso- mal “leaky scanning” mechanism for the synthesis of the envelope glycoproteins (Gn and Gc), a cytosolic accessory protein (NSm) that can be found in two isoforms of 13–14-kDa protein [6], and a 78-kDa glycoprotein (NSm-Gn) that incorporates in virus particles when produced in insect cells [7] but with unknown functions in mammal hosts. The S segment encodes in an ambisense strategy the viral 27 kDa nucleoprotein (N), and a 30kDa protein (NSs), considered the main virulence factor of the virus. Viruses 2021, 13, 542. https://doi.org/10.3390/v13040542 https://www.mdpi.com/journal/viruses