Role of matrix and fusion proteins in budding of Sendai virus

Paramyxoviruses are assembled at the surface of infected cells, where virio ns are formed by the process of budding. We investigated the roles of three Sendai virus (SV) membrane proteins in the production of virus-like partic les. Expression of matrix (NI) proteins from cDNA induced the budding and r elease of virus-like particles that contained M, as was previously observed with human parainfluenza virus type I (hPIV1). Expression of SV fusion (F) glycoprotein from cDNA caused the release of virus-like particles bearing surface F, although their release was less efficient than that of particles bearing NI protein. Cells that expressed only hemagglutinin-neuraminidase (HN) released no HN-containing vesicles. Coexpression of M and F proteins e nhanced the release of F protein by a factor greater than 4. The virus-like particles containing F and NI were found in different density gradient fra ctions of the media of cells that coexpressed M and F, a finding that sugge sts that the two proteins formed separate vesicles and did not interact dir ectly. Vesicles released by M or F proteins also contained cellular actin; therefore, actin may be involved in the budding process induced by viral M or F proteins. Deletion of C-terminal residues of NI protein, which has a s equence similar to that of an actin-binding domain, significantly reduced r elease of the particles into medium. Site-directed mutagenesis of the cytop lasmic tail of F revealed two regions that affect the efficiency of budding : one domain comprising five consecutive amino acids conserved in SV and hP IV1 and one domain that is similar to the actin-binding domain required for budding induced by M protein. Our results indicate that both M and F prote ins are able to drive the budding of SV and propose the possible role of ac tin in the budding process.