THE BEL1 PROTEIN OF HUMAN FOAMY VIRUS CONTAINS ONE POSITIVE AND 2 NEGATIVE CONTROL REGIONS WHICH REGULATE A DISTINCT ACTIVATION DOMAIN OF 30 AMINO-ACIDS

The Bell transactivator is essential for the replication of human foamy virus (HFV). To define the functional domains of HFV Bell, we generated random missense mutations throughout the entire coding sequence of Bell. Functional analyses of 24 missense mutations have revealed the presence of at least two functional domains in Bell. One domain corresponds to a basic amino acid-rich motif which nets as a bipartite nuclear targeting sequence. A second, central domain corresponds to a presumed effector region which, when mutated, leads to dominant-negative mutants and/or lacks transactivating ability. In addition, deletion analyses and domain-swapping experiments further showed that Bell protein contains a strong carboxy-terminal activation domain. The activating region is also capable of functioning as a transcription-activating domain in yeast cells, although it does not bear any significant sequence homology to the well-characterized acidic activation domain which is known to function only in yeast and mammalian cells. We also demonstrated that the regions of Bell from residues 1 to 76 and from residues 153 to 225 repressed transcriptional activation exerted by the Bell activation domain. In contrast, the region from residues 82 to 150 appears to overcome an inhibitory effect. These results indicate that Bell contains one positive and two negative regulatory domains that modulate a distinct activation domain of Bell. These regulatory domains of Bell cannot affect the function of the VP16 activation domain, suggesting that these domains specifically regulate the activation domain of Bell. Furthermore, in vivo competition experiments showed that the positive regulatory domain acts in trans. Thus, our results demonstrate that Bell-mediated transactivation appears to undergo a complex regulatory pathway which provides a novel mode of regulation for a transcriptional activation domain.