Identification of a TAL1 Target Gene Reveals a Positive Role for the LIM Domain-Binding Protein Ldb1 in Erythroid Gene Expression and Differentiation

Abstract

The TAL1 (or SCL) gene, originally identified from its involvement by a recurrent chromosomal translocation, encodes a basic helix-loop-helix transcription factor essential for erythropoiesis. Although presumed to regulate transcription, its target genes are largely unknown. We show here that a nuclear complex containing TAL1, its DNA-binding partner E47, zinc finger transcription factor GATA-1, LIM domain protein LMO2, and LIM domain-binding protein Ldb1 transactivates the protein 4.2 (P4.2) gene through two E box GATA elements in its proximal promoter. Binding of this complex to DNA was dependent on the integrity of both E box and GATA sites and was demonstrated to occur on the P4.2 promoter in cells. Maximal transcription in transiently transfected cells required both E box GATA elements and expression of all five components of the complex. This complex was shown, in addition, to be capable of linking in solution double-stranded oligonucleotides corresponding to the two P4.2 E box GATA elements. This DNA-linking activity required Ldb1 and increased with dimethyl sulfoxide-induced differentiation of murine erythroleukemia (MEL) cells. In contrast, enforced expression in MEL cells of dimerization-defective mutant Ldb1, as well as wild-type Ldb1, significantly decreased E box GATA DNA-binding activities, P4.2 promoter activity, and accumulation of P4.2 and ␤-globin mRNAs. These studies define a physiologic target for a TAL1-and GATA-1-containing ternary complex and reveal a positive role for Ldb1 in erythroid gene expression and differentiation.Red blood cell production is controlled by the concerted actions of signal transducers and nuclear regulators. Genetargeting studies have demonstrated a requirement for several nuclear proteins in erythropoiesis, including the TAL1 (or SCL) and GATA-1 transcription factors and the LIM domain protein LMO2. These proteins, singly and in combination, bind to specific promoter and enhancer elements to regulate the expression of genes that execute the differentiation program.

Modularity of CHIP/LDB transcription complexes regulates cell differentiation

“…58 Humans with inherited mutations in this gene and mice homozygous for a targeted mutation of the locus exhibit defective erythrocyte ion transport and spherocytic anemia. [59][60][61] the Dna-binding activity of the LDB-Gata complex and resultant P4.2 gene expression increase with erythroid differentiation 44 and are very sensitive to the levels of ssDP2. 45 thus, the balance between the LDB-LHX and the LDB-Gata complexes is important for normal differentiation of erythrocytes, similarly to our observations for the fly macrochaetae.…”

“…the constituents, assembly and function of CHIP/LDB complexes are highly conserved through evolution. Complexes containing either ssDP-LDB-LHX proteins, analogous to the fly CHIP-aP complex [37][38][39][40][41] or ssDP-LDB-Gata proteins, analogous to the Drosophila CHIP-Pnr complex [42][43][44][45] ( Fig. 1) have been isolated from mammalian tissues and their transcriptional activity has been amply demonstrated.…”

SSBP2, a candidate tumor suppressor gene, induces growth arrest and differentiation of myeloid leukemia cells

“…LMO2, originally isolated due to its activation in T-cell leukemia, is a key regulator of normal hematopoietic differentiation. More recent evidence suggests that LMO2 acts as a bridge in assembling a multiprotein transcription complex comprising tal-1, ldb1 and GATA-1 in erythroid cells (Wadman et al, 1997;Xu et al, 2003).…”

Mechanism governing a stem cell-generating cis -regulatory element

“…The GATA and E-box motifs, and the spacing between the motifs, are essential for +9.5 site enhancer activity in reporter assays (11). The E-box binding protein TAL1 cooperates with GATA factors in the assembly of a multicomponent complex on E-box-GATA composite elements at genes important for blood cell development and function (27)(28)(29)(30)(31)(32)(33). The TAL1-interacting proteins LDB1 and LMO2 control the development and function of HSPCs (22,(34)(35)(36)(37)(38).…”