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Myogenic differentiation can be inhibited by the adenovirus E1a protein in the rat L6 muscle cell line. The present investigation provides evidence that E1a interferes with the expression of myogenin and the activity of Myf-5, the two myogenic helix-loop-helix (HLH) proteins that are expressed in L6 muscle cells. In nuclei of E1a-expressing L6 cells, Myf-5 protein accumulates to normal or even elevated levels and shows no alterations of its ability to bind to the DNA-binding site (CANNTG). However, trans-activation of muscle-specific reporter genes by Myf-5 is strongly inhibited. The same inhibition by E1a can be shown for the other myogenic HLH proteins, MyoD, myogenin, and MRF4/Myf-6, that have been expressed in 10T1/2 fibroblasts. In contrast to the normal level of Myf-5 expression, synthesis of myogenin is entirely abolished in the differentiation-defective L6-E1a cells. Here, we demonstrate that the carboxy-terminal trans-activator domain and probably the basic-HLH (bHLH) region of Myf-5 constitute targets for the inhibition by E1a. The effect of E1a depends on its intact transforming regions but not on the transcriptional activator domain. Our data suggest that activation of myogenin gene expression and the establishment of the differentiated phenotype may require functional Myf-5. Expression of the Myf-5 gene, however, is apparently independent of auto- or cross-regulation by the myogenic HLH proteins.

Inhibition of muscle differentiation by the adenovirus E1a protein: repression of the transcriptional activating function of the HLH protein Myf-5.