Myocyte-Specific Enhancer Factor 2A
Created by Brian Muffly
The myocyte-specific enhancer factor 2A (MEF2A, PDB ID: 3KOV) is a transcription factor. MEF2A belongs to the myocyte enhancer factor 2 (MEF2) family of proteins. The vertebrate genome contains four different MEF2 genes (A, B, C, and D) that play critical roles in cellular development, regulation, and survival, among others (1). MEF2 is modulated by posttranslational modifications and interactions with other proteins that occur in one of the protein’s two major domains: the MADS-box and the MEF2-specific domain. These two domains comprise the highly conserved N-terminal region of the protein, and function in both DNA-binding and protein-protein interactions (2). The MADS-box includes the first 58 residues. Stabilized by extensive interactions with the MADS-box and DNA, the following MEF2-specific domain is unique to the protein family. It is responsible for recognizing a particular consensus DNA sequence (1). The entire MADS-box/MEF2 domain forms a stable folded structure upon DNA binding, and largely preserves this structure upon ligand binding (1). Meanwhile, the remaining evolutionarily divergent C-terminus is involved primarily in transcriptional activation and nuclear localization (3).
Specifically, MEF2A regulates gene expression in a variety of developmental capacities and adaptive responses (1). With 360 total residues, a single MEF2A molecule is comprised of four chains, or monomers, of 90 amino acids each. Each monomer includes an extended N-terminal tail, three alpha helices (H1, H2, and H3), and three beta strands (B1, B2, and B3) (1). MEF2 is typically found as a dimer, in which the N-terminal tail and helix H1 form the major DNA binding surface. To accomplish binding, numerous N-terminal residues (Gly-2, Arg-3, Lys-4, Lys-5, Ile-6) are inserted into the minor groove of the DNA strand (1).
Structural analysis indicates that the MADS-box and MEF2-specific domain exist as an intertwined, folded structure rather than as two distinct domains (1). The stability experienced by the MEF2-specific domain can be attributed to several intrinsic folding interactions. First, hydrogen bonds exist between Ser-78 of one monomer and Gln-56 of its neighboring monomer. Also, several residues of H3 (Ile-84, Val-85, Leu-88) experience hydrophobic interactions with residues from H1 (Phe-26, Met-29, Lys-30, Tyr-33) of the adjacent monomer. The two H2 helices, along with H3 and B3, serve as binding sites for proteins interacting with MEF2 (1). Interactions between B2 and B3, as well as H1 and H3, help hold the dimer together. Additionally, His-76 and Asp-63 are thought to play major roles in the function of MEF2A. Although it has not been shown experimentally, it is thought that a point mutation of these residues could alter the binding of various cofactors to MEF2A (1). The side chain orientations of His-76, Tyr-72, Leu-54, and Gln-56 create the co-factor binding pockets within MEF2A.
One function of MEF2A is its role in regulating the expression of the glucose transporter isoform 4 (GLUT4) gene (4). Understanding the role of MEF2A in transcriptional regulation of GLUT4 could provide a point of intervention into the treatment and management of insulin resistant diabetes. Found in adipose tissue and muscle cells, GLUT4 is an insulin-regulated transporter responsible for the translocation of glucose inside of the cell (5). During exercise, in which there is an increased demand for glucose, certain proteins increase binding in two conserved regions on the GLUT4 gene promoter (4). This binding activates gene transcription. One region is bound by a MEF2A/D heterodimer, while the other is bound by a transcription factor, GLUT4 enhancer factor (GEF) (5). It is thought that transcriptional repressors like HDAC5 dissociate from MEF2 prior to transcriptional activation, but further research is needed to determine the signaling events mediating such a process (4). Although the mechanism is unknown, others believe MEF2A and GEF function cooperatively in the regulation of GLUT4 transcription (5). The binding of MEF2A to DNA, HDAC5, and/or GEF illustrates the connection between the structure and function of MEF2A. As mentioned, MEF2A DNA binding is facilitated by the N-terminal tail and H1, while ligand binding is primarily mediated by the H2 helices of MEF2A (1).
MEF2A also plays a major role in muscle-specific gene expression during myogenesis. This is accomplished through several pathways. First, an interaction between thyroid horomone receptors (TRs) and MEF2A is required to activate gene transcription in muscles (6). When MEF2A and TR functionally interact, transcription of myosin promoters is activated. This physical association with MEF2A occurs through the MADS-box. Furthermore, studies have demonstrated that this TR/MEF2A complex also combines p300 for additional transcriptional regulation (6). The p300 protein binds to the N-terminal 195-amino acid fragment of MEF2A.
Secondly, myocardin-related transcription factors (MRTFs) have been shown to interact with MEF2 proteins through the MADS-box/MEF2-specific domain (1). In particular, MTRFs attach to the exposed H2, B3, and H3 regions of the MEF2-specific domain. These interactions activate transcription of genes in cardiac and smooth muscle. MASTR, a member of the MRTF family, binds discrete hydrophobic pockets within MEF2. For example, the side chains of Leu-66, Leu-67, and Asp-63 form such a binding pocket (1). Further stability is provided by hydrogen bonds and electrostatic interactions at the MASTR-MEF2 interface. Again, the connection between MEF2’s structure and function is illustrated in its regulation of muscle-specific gene transcription.
In conclusion, the myocyte-specific enhancer factor 2A is a fascinating protein. The numerous and diverse functions of MEF2A highlight its importance in future research. The role of MEF2 in neuronal development and survival, for instance, suggests that MEF2 proteins could be targeted in the treatment of neurodegenerative and psychiatric disorders (7). Additionally, increasing evidence suggests that these proteins might be connected to cancer (8). Such results open the possibility of targeting MEF2 proteins in cancer therapy. Only as research continues will more of the puppeteer’s strings be revealed.