Abstract
Bombyx mori densovirus type 1 (BmDV) is a pathogen that causes flacherie disease in the silkworm. The absolute nonsusceptibility to BmDV among certain silkworm strains is determined independently by two genes, nsd-1 and Nid-1. However, neither of these genes has been molecularly identified to date. Here, we isolated the nsd-1 gene by positional cloning and characterized the properties of its product, NSD-1. Sequence and biochemical analyses revealed that this gene encodes a Bombyx-specific mucin-like glycoprotein with a single transmembrane domain. The NSD-1 protein was specifically expressed in the larval midgut epithelium, the known infection site of BmDV. Sequence analysis of the nsd-1 gene from 13 resistant and 12 susceptible strains suggested that a specific arginine residue in the extracellular tail of the NSD-1 protein was common among susceptible strains. Germline transformation of the susceptible-type nsd-1 (with a single nucleotide substitution) conferred partial susceptibility to resistant larvae, indicating that the + nsd-1 gene is required for the susceptibility of B. mori larvae to BmDV and the susceptibility is solely a result of the substitution of a single amino acid with arginine. Taken together, our results provide striking evidence that a novel membrane-bound mucin-like protein functions as a cell-surface receptor for a densovirus.
Highlights
Densoviruses belong to the family Parvoviridae and infect insects and crustaceans[1]
Immunohistochemical experiments using anti-Bombyx mori densovirus (BmDV) capsid antibody confirmed that BmDV propagated in the nuclei of the midgut epithelial cells of the GAL4/upstream activating sequence (UAS) line but not in the GAL4 or UAS line (Fig. 4B). These results proved that nsd-1 controls resistance to the virus, and the mucin-like membrane protein encoded by + nsd-1 is required for BmDV infection
We molecularly identified the nsd-1 gene, which potentially encodes a mucin-like single-pass membrane protein
Summary
Densoviruses belong to the family Parvoviridae and infect insects and crustaceans[1]. BmDV infects only in the columnar cells of the midgut epithelium, and multiplies in the nuclei of the infected cells. Histopathological studies on the midgut epithelium of silkworm larvae infected with BmDV revealed hypertrophic columnar cell nuclei that stained markedly with the Feulgen reaction or methyl green[5]. During BmDV infection, nsd-1 and Nid-1 block the early and late steps of virus infection in the silkworm, respectively[20]. Neither of these genes has been molecularly identified to date. Cloning of both BmDV resistance genes will provide significant information about how densoviruses can infect silkworm midgut cells. Through DNA sequencing and germline transformation, we have demonstrated that a single amino acid in NSD-1 determines the specific interaction between BmDV and silkworm midgut epithelial cells
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