A novel human muscle factor related to but distinct from MyoD1 induces myogenic conversion in 10T1/2 fibroblasts

We have isolated the cDNA encoding a novel human myogenic factor, Myf‐5, by weak cross‐hydridization to the mouse MyoD1 probe. Nucleotide sequence analysis and the identification of the corresponding gene indicate that human Myf‐5 is a member of a small gene family which also contains the human homologue to MyoD1. Although structurally related to the mouse factor, the human Myf‐5 constitutes a different protein which nevertheless is capable of inducing the myogenic phenotype in embryonic C3H mouse 10T1/2 ‘fibroblasts’. The existence of more than one MyoD1‐like protein in human skeletal muscle is further suggested by the detection of several similar but distinct cDNA clones. The phenotypic conversion of 10T1/2 cells by the human factor is recognized by the capacity of the cells to form multinucleated syncytia and synthesize sarcomeric myosin heavy chains. Moreover, transient expression of Myf‐5 in 10T1/2 cells leads to the activation of a co‐transfected muscle‐specific CAT reporter gene which by itself is transcriptionally silent in the non‐muscle cell background. The deduced amino acid sequence of clone Myf‐5 reveals a region which is highly similar to myc proteins and the developmental factors from Drosophila encoded by the achaete scute locus and the twist gene. The myc homology region and a preceding cluster of basic amino acids are located in a larger sequence domain with strong similarity to the mouse myogenic factor MyoD1. Two additional short segments with high serine and threonine content are conserved between the two proteins.