Matrix-glycoprotein interactions required for budding of a plant nucleorhabdovirus and induction of inner nuclear membrane invagination.

Nucleorhabdoviruses such as Sonchus yellow net virus (SYNV) replicate in the nuclei and undergo morphogenesis at the inner nuclear membrane (IM) in plant cells. Mature particles are presumed to form by budding of the Matrix (M) protein-nucleocapsid complexes through host IMs to acquire host phospholipids and the surface glycoproteins (G). To address mechanisms underlying nucleorhabdovirus budding, we generated recombinant SYNV G mutants containing a truncated amino-terminal (NT) or carboxyl-terminal (CT) domain. Electron microscopy and sucrose gradient centrifugation analyses showed that the CT domain is essential for virion morphogenesis whereas the NT domain is also required for efficient budding. SYNV infection induces IM invaginations that are thought to provide membrane sites for virus budding. We found that in the context of viral infections, interactions of the M protein with the CT domain of the membrane-anchored G protein mediate M protein translocation and IM invagination. Interestingly, tethering the M protein to endomembranes, either by co-expression with a transmembrane G protein CT domain or by artificial fusion with the G protein membrane targeting sequence, induces IM invagination in uninfected cells. Further evidence to support functions of G-M interactions in virus budding came from dominant negative effects on SYNV-induced IM invagination and viral infections that were elicited by expression of a soluble version of the G protein CT domain. Based on these data, we propose that cooperative G-M interactions promote efficient SYNV budding.