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Review
. 2000 Jan;20(2):429-40.
doi: 10.1128/MCB.20.2.429-440.2000.

Helix-loop-helix proteins: regulators of transcription in eucaryotic organisms

M E Massari  1 , C Murre
Affiliations
Review

Helix-loop-helix proteins: regulators of transcription in eucaryotic organisms

M E Massari et al. Mol Cell Biol. 2000 Jan.
No abstract available

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Figures

FIG. 1
FIG. 1
Multiple sequence alignment and classification of some representative members of the HLH family of transcription factors. Shown is a dendrogram created by aligning the sequences of the indicated HLH proteins by the Clustal W algorithm (160).
FIG. 2
FIG. 2
Models for transcriptional activation and repression by the HLH proteins. Shown is a schematic depiction of an HLH target gene containing an E-box element (green box) (or an N box for Hairy [H]). (A) Cell-type-specific activators (A) can directly interact with the p300/CBP HAT (37, 105, 129, 130, 145) when bound as a heterodimer with an E protein (E). This interaction may be via independent domains present in the HLH activator. (B) Two models for activation by the E proteins. (Upper panel) Recruitment of p300/CBP may be directly mediated by the bHLH and/or activation domains (37, 130). Alternatively, the association with p300/CBP could be bridged by another factor (X). (Lower panel) In the second model, DNA-bound E-protein homodimers directly recruit the SAGA HAT complex via a conserved transactivation domain (96). Note that these models are not mutually exclusive. The wavy arrow represents transcriptional activation, and Ac denotes acetylase activity of the indicated protein complex. (C) A model for transcriptional activation by c-Myc. Once bound to DNA as a heterodimer with Max, Myc can recruit HAT complexes (such as SAGA) which contain the ATM family member TRRAP. This recruitment is mediated by a direct interaction of the N terminus of c-Myc with TRRAP (indicated in red type) (54, 99). Myc has also been shown to interact with hSNF5, a component of the ATP-dependent SWI-SNF chromatin-remodeling complex (30). (D) Repression by the HLH proteins. The top two diagrams indicate two forms of passive (indirect) repression. A bHLH repressor (R) may heterodimerize with a bHLH activator and inhibit its ability to bind an E box, thus preventing transactivation of the target gene (58, 87). The formation of non-DNA-binding Id–E-protein heterodimers can inhibit bHLH activator function by titrating away the available pool of E proteins. The bottom three illustrations show active forms of repression that necessitate the recruitment of transcriptional corepressor molecules. An HLH repressor may homodimerize or form a heterodimer with an E protein and repress transcription through E-box (or N-box) elements (87, 98). It is also possible that these heterodimers occupy E-box elements but are transcriptionally inert (121). The deacetylase activity of Rpd3 is indicated in the figure as dAc. It is thought that once brought to DNA, deacetylase activity induces localized nucleosomal condensation and thus promotes a transcriptionally repressed state. Transcriptional repression by Max-Mad (Mxi1) heterodimers is mediated by recruitment of the Sin3–N-CoR–HDAC complex. The blackened region of Mad represents the Sin3 interaction domain.
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