Structure-function analysis of the Sendai virus F and HN cytoplasmic domain: different role for the two proteins in the production of virus particle.

The role of the cytoplasmic domain (cytd) of the Sendai virus HN and F glycoproteins in the process of virus assembly and budding are evaluated. Recombinant Sendai virus (rSeV) mutants are generated carrying modifications in the cytd of each of the glycoprotein separately. The modifications include increasing truncations and/or amino acid sequence substitutions. Following steady-state (35)[S]methionine/cysteine labeling of the infected cells, the virus particle production is estimated. The radioactive virions in the cell supernatants are measured relative to the extent of the infection, assessed by the intracellular N protein signal. For both the F and HN cytd truncation mutants, the largest cytd deletions lead to a 20- to 50-fold reduction in virion production. This reduction cannot be explained by a reduction of the cell surface expression of the glycoproteins. For the F protein mutants, the virions produced in reduced amount always exhibit a normal F protein composition. It is then concluded that a threshold level of F is required for SeV assembly and budding. The rate or the efficiency with which this threshold is reached up appears to depend on the nature of the F cytd. A minimal cytd length is required as well as a specific sequence. The analysis of HN protein mutants brings to light an apparent paradox. The larger cytd truncations result in significant reduction of virion production. On the other hand, a normal virion production can take place with an underrepresentation of or, even, an undetectable HN in the particles. The HN uptake in virion is confirmed to depend on the previously proposed cytd SYWST signal (T. Takimoto, T. Bousse, E. C. Coronel, R. A Scroggs, and A. Portner. 1998. J. Virol. 72, 9747-9754.).