VIRAL LIPOSOMES RELEASED FROM INSECT CELLS INFECTED WITH RECOMBINANT BACULOVIRUS EXPRESSING THE MATRIX PROTEIN OF VESICULAR STOMATITIS-VIRUS

The matrix (M) protein of vesicular stomatitis virus (VSV) has been fo und to promote assembly and budding of virions as well as down-regulat ing of VSV transcription. Large quantities of M protein can be produce d in insect cells infected with recombinant baculovirus expressing the VSV M gene under control of the polyhedrin promoter. Analysis by puls e-chase experiments and density gradient centrifugation revealed that the [S-35] methionine-labeled M protein synthesized in insect cells is released into the extracellular medium in association with lipid vesi cles (liposomes). Electron microscopy and immunogold labeling showed t hat M protein expressed in insect cells induced the formation on plasm a membrane of vesicles containing M protein, which are released from t he cell surface in the form of liposomes. The baculovirus vector itsel f or recombinants expressing VSV glycoprotein (G) or nucleocapsid (N) protein did not produce the formation of vesicles in infected cells. T he baculovirus-expressed M protein retains biological activity as demo nstrated by its capacity to inhibit transcription when reconstituted w ith VSV nucleocapsids in vitro. These data suggest that M protein has the capacity to associate with the plasma membrane of infected cells a nd, in so doing, causes evagination of the membrane to form a vesicle which is released from the cell. This observation leads to the postula te, which requires further proof, that the VSV M protein can induce th e formation and budding of liposomes from the cell membrane surface.