Double-layered membrane vesicles released from mammalian cells infected with Sendai virus expressing the matrix protein of vesicular stomatitis virus

The matrix (M) protein of vesicular stomatitis virus (VSV) was reported to form vesicles on the cell surface and subsequently to be released into the cultured medium when expressed from cDNA by virus vectors. To further inves tigate VSV M activity, we generated a recombinant Sendai virus (SeV) expres sing the VSV M protein (SeV-M-VSV). When cells were infected with SeV-M-VSV , VSV M was found abundantly in the culture medium. Electron microscopy dem onstrated the budding of two-membraned vesicles (greater than or equal to 0 .8 mu m in diameter) from the infected cells. The outer membrane of the ves icle was derived from the plasma membrane and the inner one possibly derive d from the membrane of an intracellular vesicle. Immune-gold labeling showe d that VSV M was exclusively located in a double-layered region. The releas ed membranes were divided into three parts: the VSV M Vesicles with SeV F a nd HN glycoproteins, SeV particles, and vesicles associated with the cytoso lic components. The last abundantly contained phosphorylated SeV matrix (M) protein, which is not released in a usual SeV infection. Furthermore the V SV M protein expressed without using a virus vector was efficiently release d into the culture medium. These results suggest that the VSV M protein has a budding activity per se and that SeV proteins are passively involved in the release of VSV M. (C) 1999 Academic Press.