Function and Classification
MyoD, along with Myf5, is responsible for muscle cell differentiation and establishment of the myogenic lineage. It is a member of the basic helix loop helix (bHLH) family and myogenic factors subfamily of proteins [1].
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 . The HLH domain functions in and forms homodimeric and heterodimeric complexes through a combination of hydrophobic interactions and hydrogen bonding (hydrophobic residues in gray, hydrogen bonding in purple) [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].
DNA Interaction
MyoD, along with most other bHLH proteins, recognizes the consensus DNA sequence CAN NTG, where N can be any base. This sequence is known as the and is bound by MyoD's (basic residues pictured in blue, acidic in red) in DNA's major groove. MyoD's basic region residues indirectly establish selectivity 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. The lysine and arginine residues are situated near the backbone of the DNA and interact with the backbone phosphate groups and create a type of phosphate clamp, while the alanine and threonine are in DNA's major groove and interact with the DNA's bases [4].
Regulation
MyoD is subject to regulation both by complex formation with other proteins and by DNA binding. Differences in E-box sequences, as well as sequences flanking the E-box, and in complex formation determine the transcription factor's effect and allow differentiation into a diverse array of muscle cells [5]. 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 [6].
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 [7]. 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 [8].
The protein IFRD1 is an activating cofactor of MyoD. This protein and MyoD cooperatively activate muscle-specific enhancers. This same cofactor also represses NF-κB, which has been shown to inhibit MyoD mRNA translation [9].
p300, a histone deacetyltransferase, cooperatively interacts with MyoD in the process of converting fibroblasts to myoblasts. This interaction occurs between MyoD's activation domain and both the amino terminus and carboxy terminus of p300 [10].
MyoD is 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 [11]. Ubiquination takes place only when MyoD is hyperphosphorylated at its cyclin-dependent kinase (CDK) sites. These CDK sites are Ser or Thr residues that are preceded by a Pro residue. Ser200 has been demonstrated to be required for MyoD to become hyperphosphorylated [12].
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 [13].
