MyoD, a member of the family of helix-loophelix myogenic factors that plays a crucial role in skeletal muscle differentiation, is a nuclear phosphoprotein. Using microinjection of purified MyoD protein into rat fibroblasts, we show that the nuclear import of MyoD is a rapid and active process, being ATP and temperature dependent. Two
The results reported here indicate that retinoic acid (RA) induces growth arrest and dierentiation only in MyoDexpressing muscle cells. Transient transfection assays reveal a functional interaction between MyoD, a key myogenic regulator and RA-receptors, principal mediators of RA actions. Interestingly, we demonstrate that RXR-MyoD-containing complexes are recruited at speci®c MyoD DNA-binding sites in muscle cells.Furthermore, we also demonstrate that RA-receptors and the muscle basic helix ± loop ± helix (b-HLH) proteins interact physically. Mutational analysis suggests that this interaction occurs via the basic region of muscle b-HLH proteins and the DNA-binding domain of RAreceptors and is important for functional interactions between these two families of transcription factors. In conclusion, these results highlight novel interactions between two distinct groups of regulatory proteins that in¯uence cell growth and dierentiation.
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