MULTIPLE ROLES FOR THE MYOD BASIC REGION IN TRANSMISSION OF TRANSCRIPTIONAL ACTIVATION SIGNALS AND INTERACTION WITH MEF2

Establishment of skeletal muscle lineages is controlled by the MyoD fa mily of basic helix-loop-helix (bHLH) transcription factors, The abili ty of these factors to initiate myogenesis is dependent on two conserv ed amino acid residues, alanine and threonine, in the basic domains of these factors, It has been postulated that these two residues may be responsible for the initiation of myogenesis via interaction with an e ssential myogenic cofactor, The myogenic bHLH proteins cooperatively a ctivate transcription and myogenesis through protein-protein interacti ons with members of the myocyte enhancer factor 2 (MEF2) family of MAD S domain transcription factors, MyoD-E12 heterodimers interact with ME F2 proteins to synergistically activate myogenesis, while homodimers o f E12, which lack the conserved alanine and threonine residues in the basic domain, do not interact with MEF2, We have examined whether the myogenic alanine and threonine in the MyoD basic region are required f or interaction with MEF2. Here, we show that substitution of the MyoD basic domain with that of E12 does not prevent interaction with MEF2, Instead, the inability of alanine-threonine mutants of MyoD to initiat e myogenesis is due to a failure to transmit transcriptional activatio n signals provided either from the MyoD or the MEF2 activation domain. This defect in transcriptional transmission can be overcome by substi tution of the MyoD or the MEF2 activation domain with the VP16 activat ion domain, These results demonstrate that myogenic bHLH-MEF2 interact ion can be uncoupled from transcriptional activation and support the i dea that the myogenic residues in myogenic bHLH proteins are essential for transmission of a transcriptional activation signal.