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 foam y virus (HFV). To define the functional domains of HFV Bell, we genera ted 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 corres ponds to a basic amino acid-rich motif which nets as a bipartite nucle ar targeting sequence. A second, central domain corresponds to a presu med effector region which, when mutated, leads to dominant-negative mu tants and/or lacks transactivating ability. In addition, deletion anal yses and domain-swapping experiments further showed that Bell protein contains a strong carboxy-terminal activation domain. The activating r egion is also capable of functioning as a transcription-activating dom ain 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 demonstra ted that the regions of Bell from residues 1 to 76 and from residues 1 53 to 225 repressed transcriptional activation exerted by the Bell act ivation domain. In contrast, the region from residues 82 to 150 appear s to overcome an inhibitory effect. These results indicate that Bell c ontains one positive and two negative regulatory domains that modulate a distinct activation domain of Bell. These regulatory domains of Bel l cannot affect the function of the VP16 activation domain, suggesting that these domains specifically regulate the activation domain of Bel l. Furthermore, in vivo competition experiments showed that the positi ve 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 transcription al activation domain.