Structures and DNA sequence preferences of USF1 and USF2 and proteins that interact with them. USF1 and USF2 contain nuclear localization sequences (NLS), as well as basic region (BR), HLH, and LZ domains in the CTD-specific region and a USR in the N-terminal domain. USF heterodimers preferentially associate with the E-box sequence CACGTGAC but also interact with pyrimidine-rich initiator elements in conjunction with transcription factor TFII-I. Proteins documented to interact with USF proteins are listed at the bottom (references are in brackets).

Structure and DNA sequence preferences of Tal1 and proteins that interact with it. Tal1 contains an HLH domain and a basic region (BR) in the CTD, as well as an activation domain (AD) and a nuclear localization sequence (NLS) in the N-terminal domain. The Tal1 gene contains a short upstream open reading frame (uORF). Tal1 associates with E proteins and binds to E-box elements, with a preference for the sequence CAGCTG. Tal1 also forms a large complex consisting of GATA-1, LMO2, and Ldb1 and binds to composite elements containing an E box and a GATA site separated by about 9 bp or to a GATA sequence only. The recruitment of Tal1 to CACCC motifs is likely mediated by proteins that have not been identified yet. Proteins documented to interact with Tal1 are listed at the bottom (references are in brackets).

Model outlining a correlation of erythroid cell differentiation, calcium concentration, and HLH transcription factor activity. During the differentiation of erythroid cells, there is a transient increase in the intracellular calcium concentration. This activates m-calpain, causing proteolytic cleavage of USF and Myc. The Myc cleavage product Nick, localized in the cytoplasm, may regulate processes involved in the differentiation of erythroid cells. At later erythroid cell differentiation stages, the expression of Myc is repressed. The decrease in the calcium concentration, which may or may not be regulated by the relocation of TFII-I to the cytoplasm, inhibits calpain activity, allowing increased expression of full-length USF. Tal1 expression increases during the differentiation of erythroid cells, but proteolytic degradation in erythroid progenitor cells likely facilitates the conversion of Tal1 from a repressor to an activator. If and how the expression of TFII-I changes during erythroid cell maturation is unknown. It is postulated that during the early stages of erythropoiesis, TFII-I, perhaps in conjunction with USF or Myc, functions as a repressor of erythroid cell-specific genes.

Structure and DNA sequence preferences of TFII-I and proteins that interact with it. The gene for TFII-I contains six repeat regions, each encoding an HLH domain. In addition, an LZ motif, a nuclear localization sequence (NLS), a basic region (BR), and two Ph domains are located in the C terminus. TFII-I associates with USF and interacts with pyrimidine-rich initiator elements or E-box motifs. Proteins documented to interact with TFII-I are listed at the bottom (references are in brackets).

Structure and DNA sequence preferences of Myc and proteins that interact with it. Myc contains a basic region (BR), an HLH domain, and an LZ in the CTD. Two Myc-binding elements (MB1 and MB2) and a nuclear localization sequence (NLS) are located in the N-terminal domain (NTD). Myc interacts with Max and binds to E-box motifs preferentially with CG, CA, TG, and CG residues in the center. It also interacts with genomic regions that lack E-box sequences; these associations are likely mediated by the interaction of Myc with other DNA-binding proteins. Proteins documented to interact with Myc are listed at the bottom (references are in brackets).

Hypothetical model of the regulation of an erythroid cell-specific gene by HLH proteins during the differentiation of erythroid cells. It is proposed that USF2 homodimers and Tal-1/E-protein heterodimers occupy enhancer elements of erythroid cell-specific genes in erythroid progenitor cells. At this stage, Pol II is recruited at low levels and enhancer transcription may assist in maintaining an open configuration (65, 75). The Tal1/E-protein/ETO complex and TFII-I/Myc heterodimers bind to the promoter regions and, through corepressors, keep the promoter in an inactive configuration. During the differentiation of erythroid cells, both enhancer and promoter regions are sequentially activated, which is facilitated by the following processes: (i) relocation of TFII-I to the cytoplasm, (ii) repression of Myc expression, (iii) an increase in the activity of USF, and (iv) conversion of Tal1 from a repressor to an activator. This leads to increased binding of activating HLH proteins, including USF heterodimers and Tal1/GATA-1/LMO2/Lbd1, as well as coactivators, to the enhancer and the promoter. Ldb1 and other activities then mediate proximity between the enhancer and promoter, and activities are transferred from the enhancer to the promoter to position transcription complexes, to enhance transcription elongation, and/or to facilitate reinitiation of transcription. This is a simplified scenario that, in reality, involves many other erythroid cell-specific and ubiquitous activities.