Figure 3. Structure of myocardin family members.

Functional domains of homology among the myocardin family proteins are shown and the numbers of amino acids are indicated. Myocardin-related transcription factors (MRTFs) are potent transcriptional coactivators that associate with serum response factor (SRF) through a basic region (++) and an adjacent Glu-rich domain (Q). Between these domains is a short α-helical region with similar secondary structure to a domain in the ternary complex factor protein ELK1, known as a B box, which mediates their interaction with SRF^52,68,71. Myocard in family proteins contain Arg-Pro-X-X-X-Glu-Leu (RPEL) motifs, which mediate their interact ion with globular actin (G-actin). Members of the myocardin family have a homologous SAP domain, named after SAFA or SAFB, acinus and PIAS, which participates in different kinds of chromosomal DNA metabolism. Deletion of this region disrupts the ability of myocardin to activate a subset of SRF-dependent genes⁶⁸. Myocardin and MRTFs contain powerful transcriptional activation domains (TADs) required for the stimulation of SRF activity. A dimerization motif resembling a Leu zipper mediates homo- and heterodimerization of myocardin and MRTFs. Alternative usage of 5′ exons in the myocardin gene gives rise to proteins with different amino termini. A cardiac-specific splice variant of myocard in contains a unique amino-terminal sequence that confers the ability to interact with the myocyte-specific enhancer factor 2 (MEF2) transcription factor, a MADS-boxtranscription factor related to SRF. This MEF2-interaction domain is also contained in a divergent member of the myocard in family called MEF2-activating SAP transcriptional regulator (MASTR). MASTR lacks the SRF-interaction domain.The OTT (also known as RBM1B)-MAL (also known as MRTF-A) fusion protein of AMKL leukaemia cells contains MRTF-A sequences.