We have described the application of a simple biotinylation tagging approach for 
the direct purification of tagged transcription factor complexes, based on the 
use of artificial short peptide tags that are specifically and efficiently 
biotinylated by the bacterial BirA biotin ligase, which is co-expressed in cells 
with the tagged factor. We used this approach to initially characterize 
complexes formed by the hematopoietic transcription factor GATA-1 in erythroid 
cells. GATA-1 is essential for the erythroid differentiation, its functions 
encompassing upregulation of erythroid genes, repression of alternative 
transcription programs, and suppression of cell proliferation. However, it was 
not clear how all of these GATA-1 functions are mediated. Our work describes, 
for the first time, distinct GATA-1 interactions with the essential 
hematopoietic factor Gfi-1b, the repressive MeCP1 complex, and the chromatin 
remodeling ACF/WCRF complex, in addition to the known GATA-1/FOG-1 and 
GATA-1/TAL-1 complexes. We also provide evidence that distinct GATA-1 complexes 
are associated with specific GATA-1 functions in erythroid differentiation, for 
example, GATA-1/Gfi-1b with the suppression of cell proliferation and 
GATA-1/FOG-1/MeCP1 with the repression of other hematopoietic transcription 
programs. We next applied the biotinylation tag to Ldb-1, a known partner of 
GATA-1, and characterized a number of novel interaction partners that are 
essential in erythroid development, in particular, Eto-2, Lmo4, and CdK9. Last, 
we are in the process of applying the same technology to characterize the 
factors that are bound to the suppressed gamma-globin promoter in vivo.