GATA-1 functions in erythropoiesis

GATA-1 is a DNA sequence-specific zinc finger transcription factor that is essential for the differentiation of erythroid, megakaryocytic, eosinophil and mast cell lineages. In erythropoiesis, GATA-1 functions have been associated with cell survival, upregulation of the erythroid transcription program, suppression of cell proliferation and repression of transcription programs associated with multipotentiality of progenitor cells and of other hematopoietic lineages. Many GATA-1 protein interactions have been reported and include essential hematopoietic transcription factors such as TAL-1 (and its partners Ldb1, E2A and LMO2), FOG-1, EKLF, PU.1, as well as proteins involved in chromatin remodeling and modification, such as the CBP/p300 histone acetyltransferases and the SWI/SNF complex. Despite all this evidence, important questions remain as to how can GATA-1, and other transcription factors, accommodate all these functions and interactions in erythroid cells? In addressing this question, we applied an in vivo biotinylation tagging approach to directly isolate and characterize GATA-1 complexes from nuclear extracts of erythroid cells.

Proteins co-purified with GATA-1 were identified by mass spectrometry and validated by immunoprecipitation and included the essential hematopoietic transcription factors FOG-1, TAL-1, Ldb1 and Gfi-1b that were previously reported to interact with GATA-1. In addition, we identified for the first time GATA-1 interactions with the MeCP1 and WCRF/ACF chromatin remodeling complexes. The MeCP1 complex contains histone deacetylases (HDACs) and methyl DNA binding (Mbd) proteins and is associated with transcriptional repression. The ACF/WCRF chromatin remodeling complex has been associated with both activation and repression. By using a two-step immunodepletion/immunoprecipitation approach, we showed that GATA-1 forms at least five distinct complexes with FOG-1 and, independently, with FOG-1 and the MeCP1 complex, with TAL-1 and Ldb1, with Gfi-1b and with the ACF/WCRF complex. Further characterization of these complexes by transfection assays and chromatin immunoprecipitation (ChIP) assays suggested that the distinct GATA-1 complexes target distinct subsets of target genes. Specifically, we found that the GATA-1/FOG-1/MeCP1 complex represses genes (such as GATA-2) associated with the early hematopoietic multipotential state or with alternative hematopoietic lineages (such as the eosinophilic MBP gene), the GATA-1/Gfi-1b complex represses genes associated with cell proliferation (such as myc and myb), whereas the GATA-1/TAL-1/Ldb1 and the independent GATA-1/FOG-1 complex are associated with the activation of erythroid specific genes (such as EKLF and globin). On the basis of this evidence we proposed a model whereby distinct GATA-1 subcomplexes regulate specific facets of GATA-1 functions in erythropoiesis by targeting specific, potentially distinct, subgroups of GATA-1 gene targets (Figure 1). Current work in our laboratory is aimed at identifying the transcriptional regulatory networks regulated by the GATA-1 distinct subcomplexes in erythropoiesis.