Envelope viruses maturate by macromolecule assembly and budding. To investigate these steps, we generated virus-like particles (VLPs) by co-expression of structural proteins of Sendai virus (SeV), a prototype of the family Paramyxoviridae. Simultaneous expression of matrix (M), nucleo- (N), fusion (F), and hemagglutinin-neuraminidase (HN) proteins resulted in the generation of VLPs that had morphology and density similar to those of authentic virus particles, although the efficiency of release from cells was significantly lower than that of the virus. By using this VLP formation as a model of virus budding, roles of individual proteins in budding were investigated. The M protein was a driving force of budding, and the F protein facilitated and the HN protein suppressed VLP release. Either of the glycoprotenis, F or HN, as well as the N protein, significantly shifted density of VLPs to that of virus particles, suggesting that vital proteins bring about integrity of VLPs by protein-protein interactions. We further found that co-expression of a nonstructural protein, C, but not V, enhanced VLP release to a level comparable to that of virus particles, demonstrating that the C protein plays a role in virus budding. (C) 2004 Elsevier Inc. All rights reserved.