Structure
MyoD has a basic region at its amino-terminal end, which functions in binding the transcription factor to a region of the DNA known as the E-box. At the carboxyl-terminal end is MyoD's HLH domain. The HLH domain functions in protein-protein interactions and forms homodimeric and heterodimeric complexes [2].MyoD also contains an acidic activation domain. The activity of this activation domain has been observed to increase drastically upon deletion of residues in other parts of the protein. This suggests that the acidic activation domain is buried within the protein in vivo and can be activated by subtle changes in structure [3]. MyoD's ability to activate endogenous genes has been shown to rely on two regions. The first is a region rich in cysteine and histidine residues that is between the acidic activation domain and the bHLH domain. The second is a region near the carboxyl terminus of the protein. These regions are conserved in proteins with shared functionality 4.
Regulation
MyoD is subject to regulation at both its bHLH domain and its acidic activation domain. Differences in E-box sequences and in complex formation determine the transcription factor's effect and allow differentiation into a diverse array of muscle cells 2. MyoD is only functional when bound to DNA. It has been proposed that DNA binding, with its accompanying structural changes, is required in vivo to free the acidic activation domain and activate MyoD's myogenic functions 3. MyoD functions as a transcriptional activator only as a heterodimer with E proteins, which are a sub-family of bHLH proteins. This interaction takes place in the bHLH domain of both proteins. In one experiment, forced binding of E12 to MyoD that had been inhibited using E protein fragments substantially restored MyoD's activity 5. The myogenic ability of MyoD is inhibited by the presence of another bHLH protein known as Twist. Twist inhibits MyoD by competitively binding E proteins and preventing MyoD-E protein heterodimers from forming 6.
MyoD can be degraded by ubiquination of its N-terminal Lys residue. Data suggests that this occurs through attachment of ubiquitin at the N-terminal residue, followed by synthesis of a polyubiquitin chain on an internal Lys residue, which sufficiently disrupts MyoD's structure to cause degradation. This process is a major pathway of selective protein degradation in eukaryotic cells [4].
DNA Interaction
MyoD, along with most other bHLH proteins, recognizes the concensus DNA sequence CAN NTG, where N can be any base. This sequence is known as the E-box and is bound by MyoD's in DNA's major groove. MyoD's basic region residues indirectly establish specificity for specific E-box sequences by influencing the conformation in which the basic region binds DNA. There are responsible for the DNA interaction that provides MyoD's myogenic effect: Arg111, Ala114, Thr115, and Lys124 [5].
Knockout Effects
Knockout mutations of the MyoD gene have been shown to produce no distinct skeletal muscle phenotype due to an increase in Myf5 activation. Mutants lacking both MyoD and Myf5 fail to develop skeletal musculature all together [6].
