AIP1/Alix is a binding partner of Sendai virus C protein and facilitates virus budding

The C protein, an accessory protein of Sendai virus (SeV), has anti-interferon capacity and suppresses viral RNA synthesis. In addition, it is thought that the C protein is involved in virus budding because of the low efficiency of release of progeny virions from C-knockout virus-infected cells and because of the requirement of the C protein for efficient release of virus-like particles. Here, we identified AIP1/Alix, a host protein involved in apoptosis and endosomal membrane trafficking, as an interacting partner of the C protein using a yeast two-hybrid system. The amino terminus of AIP1/Alix and the carboxyl terminus of the C protein are important for the interaction in mammalian cells. Mutant C proteins unable to bind AIP1/Alix failed to accelerate the release of virus-like particles from cells. Furthermore, overexpression of AIP1/Alix enhanced SeV budding from infected cells in a C-protein-dependent manner, while the release of nucleocapsid-free empty virions was also enhanced. Finally, AIP1/Alix depletion by small interfering RNA resulted in suppression of SeV budding. The results of this study suggest that AIP1/Alix plays a role in efficient SeV budding and that the SeV C protein facilitates virus budding through interaction with AIP1/Alix.