An easy pipeline for one-step purification of SARS-CoV-2 nucleocapsid protein from insect cell suspension culture.

The COVID-19 pandemic has demanded a range of biotechnological products for detection of SARS-CoV-2 variants and evaluation of human seroconversion after infection or vaccination. In this work, we describe an easy pipeline for expression of SARS-CoV-2 nucleocapsid (N) protein in insect cells followed by its purification via affinity chromatography. The N gene was cloned into the genome of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) via transposition and the resulting recombinant baculovirus was used for infection of lepidopteran Sf9 cells adapted to high-density suspension. Using Tris-HCl pH 8.0 buffer as mobile phase and eluting bound proteins with 175 mM imidazole as part of a three-step gradient, an average of 1 mg N protein could be purified from each 50 mg of total protein from clarified supernatant. Such protein amount allows the manufacturing of serological tests and the development of basic studies on cellular responses to SARS-CoV-2.

Chikungunya virus produced by a persistently infected mosquito cell line comprises a shorter genome and is non-infectious to mammalian cells.

Although RNA viruses have high mutation rates, host cells and organisms work as selective environments, maintaining the viability of virus populations by eliminating deleterious genotypes. In serial passages of RNA viruses in a single cell line, most of these selective bottlenecks are absent, with no virus circulation and replication in different tissues or host alternation. In this work, Aedes aegypti Aag-2 cells were accidentally infected with Chikungunya virus (CHIKV) and Mayaro virus (MAYV). After numerous passages to achieve infection persistency, the infectivity of these viruses was evaluated in Ae. albopictus C6/36 cells, African green monkey Vero cells and primary-cultured human fibroblasts. While these CHIKV and MAYV isolates were still infectious to mosquito cells, they lost their ability to infect mammalian cells. After genome sequencing, it was observed that CHIKV accumulated many nonsynonymous mutations and a significant deletion in the coding sequence of the hypervariable domain in the nsP3 gene. Since MAYV showed very low titres, it was not sequenced successfully. Persistently infected Aag-2 cells also accumulated high loads of short and recombinant CHIKV RNAs, which seemed to have been originated from virus-derived DNAs. In conclusion, the genome of this CHIKV isolate could guide mutagenesis strategies for the production of attenuated or non-infectious (to mammals) CHIKV vaccine candidates. Our results also reinforce that a paradox is expected during passages of cells persistently infected by RNA viruses: more loosening for the development of more diverse virus genotypes and more pressure for virus specialization to this constant cellular environment.

Low virus diversity and spread in wild Capsicum spp. accessions from Ecuador under natural inoculum pressure.

Challenging wild plant accessions with pathogens is an initial approach for finding resistance genes for breeding programs. Viruses can be transmitted artificially by mechanical or arthropod-borne inoculation, but these experimental assays do not always reproduce natural conditions in the field. In this study, 56 wild Capsicum spp. accessions from Ecuador that were under natural inoculum pressure for six months were screened for virus infections by RNA sequencing. These plants exhibited low virus diversity in comparison to a commercial pepper cultivar that was used as a susceptible host. Subjecting numerous plants to natural infection prior to artificial assays may indicate promising accessions to track within virus/vector resistance breeding programs.

Tobamoviruses of two new species trigger resistance in pepper plants harbouring functional L alleles.

Tobamoviruses are often referred to as the most notorious viral pathogens of pepper crops. These viruses are not transmitted by invertebrate vectors, but rather by physical contact and seeds. In this study, pepper plants displaying mild mottle and mosaic symptoms were sampled in four different regions of Peru. Upon double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) tests, seven samples cross-reacted weakly with antibodies against pepper mild mottle virus (PMMoV), suggesting the presence of tobamoviruses. When employing RT-PCR, conserved primers amplified cDNA fragments of viruses from two putative new tobamovirus species in the samples. The complete genome of two representative isolates were, therefore, sequenced and analysed in silico. These viruses, which were tentatively named yellow pepper mild mottle virus (YPMMoV) and chilli pepper mild mottle virus (CPMMoV), shared highest nucleotide genome sequence identities of 83 and 85 % with bell pepper mottle virus (BpeMV), respectively. Mechanical inoculation of indicator plants with YPMMoV and CPMMoV isolates did not show any obvious differences in host ranges. These viruses were also inoculated mechanically on pepper plants harbouring different resistance L alleles to determine their pathotypes. Pepper plants carrying unfunctional L alleles (L0) to tobamoviruses were infected by all isolates and presented differential symptomatology for YPMMoV and CPMMoV. On the other hand, pepper plants carrying L1, L2, L3 and L4 alleles were resistant to all isolates, indicating that these viruses belong to pathotype P0.

The Sw-5 Gene Cluster: Tomato Breeding and Research Toward Orthotospovirus Disease Control.

The Sw-5 gene cluster encodes protein receptors that are potentially able to recognize microbial products and activate signaling pathways that lead to plant cell immunity. Although there are several Sw-5 homologs in the tomato genome, only one of them, named Sw-5b, has been extensively studied due to its functionality against a wide range of (thrips-transmitted) orthotospoviruses. The Sw-5b gene is a dominant resistance gene originally from a wild Peruvian tomato that has been used in tomato breeding programs aiming to develop cultivars with resistance to these viruses. Here, we provide an overview starting from the first reports of Sw-5 resistance, positional cloning and the sequencing of the Sw-5 gene cluster from resistant tomatoes and the validation of Sw-5b as the functional protein that triggers resistance against orthotospoviruses. Moreover, molecular details of this plant-virus interaction are also described, especially concerning the roles of Sw-5b domains in the sensing of orthotospoviruses and in the signaling cascade leading to resistance and hypersensitive response.