THE NONSTRUCTURAL NSM PROTEIN OF TOMATO SPOTTED WILT VIRUS INDUCES TUBULAR STRUCTURES IN PLANT AND INSECT CELLS

The expression and subcellular location of the 33.6-kDa nonstructural protein NSm of tomato spotted wilt virus (TSWV) was analyzed in Nicoti ana rustica plants and protoplasts as a function of time. Immunofluore scent studies in protoplasts isolated from TSWV-infected N. rustica le aves showed that this protein could first be detected close to the per iphery of the cell, near the plasmamembrane, and later in tubular stru ctures emerging from the cell surface. In situ, these tubules appeared specifically in the plasmodesmata, suggesting their involvement in ce ll-to-cell movement of the virus during systemic infection. In protopl asts transfected with an expression vector containing the NSm gene, si milar tubules were formed, indicating that NSm has the ability to form these structures in the absence of other virus-specific components. T o test whether plant-specific components were involved in tubule forma tion, the NSm gene was also expressed in a heterologous expression sys tem, i.e., insect cells. Spodoptera frugiperda and Trichoplusia ni cel ls were infected with a recombinant baculovirus expressing the NSm-gen e (AcNPV/NSm). The efficient formation of NSm-containing tubules emerg ing from the surface of both cell types indicate that no plant-specifi c cell structures or proteins are involved in their development. (C) 1 995 Academic Press, Inc.