The bet gene of feline foamy virus is required for virus replication.

Foamy viruses (FV) are complex retroviruses with additional bel genes located between env and the 3' long-terminal repeat. The functions of the bel 2 and bet genes are unknown and both are dispensable for replication of the prototypic human foamy virus in cell cultures. We examined the function(s) of bel 2 and bet of the distantly related feline foamy virus (FFV) in the proviral context. Mutagenesis was used to alter the Bel 2 and Bet or to abrogate their expression. The Bel 2/Bet mutants showed a 1000-fold reduced viral titer in feline kidney cells; in human 293T cells, viral titer was only about 10-fold reduced compared to wild-type FFV. In both cell types, the Bel 2/Bet mutations resulted in a reduced release of FFV particles. The results indicate that FFV Bet is required for efficient virus replication. The functions of the Bel 2 and Bet proteins are discussed. Copyright 2001 Academic Press.