Establishment of distinct MyoD, E2A, and twist DNA binding specificities by different basic region-DNA conformations

Mutations in the basic domain and the loop-helix II junction of TWIST abolish DNA binding in Saethre-Chotzen syndrome

Saethre-Chotzen syndrome is an autosomal dominant skull disorder resulting from premature fusion of coronal sutures (craniosynostosis). It is caused by mutations in the TWIST gene encoding a basic Helix-Loop-Helix transcription factor. Here we report on the identification of a novel mutation affecting a highly conserved residue of the basic domain. Unlike nonsense and missense mutations lying within helices, this mutation does not affect protein stability or heterodimerisation of TWIST with its partner E12. However, it does abolish TWIST binding capacity to a target E-box as efficiently as two missense mutations in the loop-helix II junction. By contrast, elongation of the loop through a 7 amino acid insertion appears not to hamper binding to the DNA target. We conclude that loss of TWIST protein function in Saethre-Chotzen patients can occur at three different levels, namely protein stability, dimerisation, and DNA binding and that the loop-helix II junction is essential for effective protein-DNA interaction.

The mesoderm specification factor twist in the life cycle of jellyfish

The basic helix-loop-helix (bHLH) transcription factor Twist is highly conserved from Drosophila to vertebrates and plays a major role in mesoderm specification of triploblasts. The presence of a Twist homologue in diploblasts such as the cnidarian Podocoryne carnea raises questions on the evolution of mesoderm, the third cell layer characteristic for triploblasts. Podocoryne Twist is expressed in the early embryo until the myoepithelial cells of the larva differentiate and then again during medusa development. There, the gene is detected first when the myoepithelial cells of the polyp dedifferentiate to form the medusa bud and later Twist is found transiently in the entocodon, a mesoderm-like cell layer which differentiates into the smooth muscle and striated muscle of the bell. On the other hand, in later bud stages and the medusa, expression is seen where non-muscle tissues differentiate. Experimental analysis of in vitro transdifferentiation and regeneration demonstrates that Twist activity is not needed when isolated striated muscle regenerate medusa organs. Developmental roles of Twist are discussed with respect to early animal evolution from a common ancestor of cnidarians and bilaterians.

Constitutive instability of muscle regulatory factor Myf5 is distinct from its mitosis-specific disappearance, which requires a D-box-like motif overlapping the basic domain

Transcription factors Myf5 and MyoD play critical roles in controlling myoblast identity and differentiation. In the myogenic cell line C2, we have found that Myf5 expression, unlike that of MyoD, is restricted to cycling cells and regulated by proteolysis at mitosis. In the present study, we have examined Myf5 proteolysis through stable transfection of myogenically convertible U20S cells with Myf5 derivatives under the control of a tetracycline-sensitive promoter. A motif within the basic helix-loop-helix domain of Myf5 (R93 to Q101) resembles the "destruction box" characteristic of substrates of mitotic proteolysis and thought to be recognized by the anaphase-promoting complex or cyclosome (APC). Mutation of this motif in Myf5 stabilizes the protein at mitosis but does not affect its constitutive turnover. Conversely, mutation of a serine residue (S158) stabilizes Myf5 in nonsynchronized cultures but not at mitosis. Thus, at least two proteolytic pathways control Myf5 levels in cycling cells. The mitotic proteolysis of Myf5 is unlike that which has been described for other destruction box-dependent substrates: down-regulation of Myf5 at mitosis appears to precede that of known targets of the APC and is not affected by a dominant-negative version of the ubiquitin carrier protein UbcH10, implicated in the APC-mediated pathway. Finally, we find that induction of Myf5 perturbs the passage of cells through mitosis, suggesting that regulation of Myf5 levels at mitosis may influence cell cycle progression of Myf5-expressing muscle precursor cells.

Regulation of muscle regulatory factors by DNA-binding, interacting proteins, and post-transcriptional modifications

Skeletal muscle differentiation is influenced by multiple pathways, which regulate the activity of myogenic regulatory factors (MRFs)-the myogenic basic helix-loop-helix proteins and the MEF2-family members-in positive or negative ways. Here we will review and discuss the network of signals that regulate MRF function during myocyte proliferation, differentiation, and post-mitotic growth. Elucidating the mechanisms governing muscle-specific transcription will provide important insight in better understanding the embryonic development of muscle at the molecular level and will have important implications in setting out strategies aimed at muscle regeneration. Since the activity of MRFs are compromised in tumors of myogenic derivation-the rhabdomyosarcomas-the studies summarized in this review can provide a useful tool to uncover the molecular basis underlying the formation of these tumors.