Conformation and Stability of the Helix-Loop-Helix Domain of the Id Protein Family

摘要

The Id proteins (inhibitors of DNA binding and cell differentiation Idl-Id4) are negative regulators of gene expression and key players in many biological processes, including angio-, neuro- and organogenesis. Moreover, they are crucial in the development of vascular diseases and cancer [1]. These small proteins (13-18 kDa) share a highly conserved HLH (helix-loop-helix) structural domain that is flanked by non-conserved TV- and C-terminal regions. The HLH motif is required for protein-protein interaction. The TV-terminus contains a phosphorylation site at Ser-5. A so-called destruction box (D-box) is present within the C-terminus of Id2, which is recognized by the anaphase promoting complex and triggers Id2 degradation [2]. Additionally, a nuclear export signal has been identified within the C-terminus of Id2, which is important for a nuclear receptor-dependent transport of the protein from the nucleus to the cytoplasm [3]. A proline-rich region C-terminal to the HLH motif is unique for Id3. Contrarily to the parent bHLH transcription factors [4], the Id proteins lack the DNAbinding basic region TV-terminal to the HLH domain. Consequently, unlike the homo/heterodimers of two bHLH proteins, bHLH-Id-protein dimers are unable to form a ternary complex with the DNA: therefore, the Id proteins not only sequester positive bHLH factors, like the ubiquitous E proteins, into inactive (non-DNA binding) dimers, but also prevent their association with tissue-specific bHLH factors, like MyoD, which would lead to activation of DNA transcription. As the highly conserved Id HLH motif is essential for protein sequestration, we are interested in the characterization and modulation of its folding and dimerization.