The role of Tomato spotted wilt virus (TSWV) glycoproteins in virus acquisition by thrips
Whitfield AE
Department of Entomology, University of Wisconsin, Madison WI 53706.
Correspondence: aew@plantpath.wisc.edu
Viruses in the family Bunyaviridae encode envelope glycoproteins, GN and GC, which play essential roles in virus entry into host cells. One of the primary objectives of my research is to characterize the individual roles of Tomato spotted wilt virus (TSWV) GN and GC during early infection events of the thrips midgut. To this end, I expressed and purified a soluble, recombinant form of the GN protein (GN-S) to directly test the hypothesis that GN plays a role in TSWV acquisition by thrips. GN-S bound larval thrips midguts without the assistance of other viral proteins when fed to thrips in an in vivo binding assay ((Whitfield AE et al. 2004. Journal of Virology 78: 13197¨C13206). In addition, GN-S reduced larval acquisition of TSWV as revealed by immunolabeling of larval midguts (Fig. 1). To test the hypothesis that GN-S inhibits virus transmission by thrips, insects were given an acquisition access period (AAP) on TSWV and GN-S or TSWV alone and then tested for their ability to transmit TSWV to leaf discs. Fifty-six percent of thrips given an AAP on TSWV transmitted virus; however, only 6% of thrips given an AAP on the combination treatment of TSWV and GN-S transmitted virus. This is the first description of transmission inhibition by a glycoprotein of a bunyavirus. Vertebrate-infecting members of the Bunyaviridae enter host cells by pH-dependent endocytosis, and during this process, the glycoproteins are exposed to conditions of acidic pH within endocytic vesicles causing the GC protein to change conformation. To gain further insight into the role of the TSWV glycoproteins in virus acquisition, virions were subjected to varying pH conditions. TSWV GC was cleaved at acidic pH indicating that it undergoes a pH-dependent conformational change consistent with it serving as a fusion protein during pH-dependent endocytosis (Whitfield AE et al. 2005. Virus Research 110: 183¨C186) GC also contains hydrophobic domains that may serve as fusion peptides during virus- and host-membrane fusion. Based on these findings, I hypothesize that TSWV enters thrips midgut cells in a pH-dependent manner and that GN serves as a viral attachment protein while GC functions as a fusion protein.
