Multiprotein bridging factor 1 (MBF1) is a coactivator which mediates trans
criptional activation by interconnecting the general transcription factor T
ATA element-binding protein and gene-specific activators such as the Drosop
hila nuclear receptor FTZ-F1 or the yeast basic leucine zipper protein GCN4
, The human homolog of MBF1 (hMBF1) has been identified but its function, e
specially in transcription, remains unclear. Here we report the cDNA clonin
g and functional analysis of hMBF1. Two isoforms, which we term hMBF1 alpha
and hMBF1 beta, have been identified. hMBF1 alpha mRNA was detected in a n
umber of tissues, whereas hMBF1 beta exhibited tissue-specific expression.
Both isoforms bound to TBP and Ad4BP/SF-1, a mammalian counterpart of FTZ-F
1, and mediated Ad4BP/SF-1-dependent transcriptional activation. While hMBF
1 was detected in the cytoplasm by immunostaining, coexpression of the nucl
ear protein Ad4BP/SF-1 with hMBF1 induced accumulation of hMBF1 in the nucl
eus, suggesting that hMBF1 is localized in the nucleus through its binding
to Ad4BP/SF-1. hMBF1 also bound to ATF1, a member of the basic leucine zipp
er protein family, and mediated its activity as a transcriptional activator
. These data establish that the coactivator MBF1 is functionally conserved
in eukaryotes.