MOS ACTIVATES MYOGENIC DIFFERENTIATION BY PROMOTING HETERODIMERIZATION OF MYOD AND E12 PROTEINS

The activities of myogenic basic helix-loop-helix (bHLH) factors are r egulated by a number of different positive and negative signals. Exten sive information has been published about the molecular mechanisms tha t interfere,vith the process of myogenic differentiation, but little i s known about the positive signals. We previously showed that overexpr ession of rat Mos in C2C12 myoblasts increased the expression of myoge nic markers whereas repression of Mos products by antisense RNAs inhib ited myogenic differentiation. In the present work, our results show t hat the rat mos proto-oncogene activates transcriptional activity of M yoD protein. In transient transfection assays, Mos promotes transcript ional transactivation by MyoD of the muscle creatine kinase enhancer a nd/or a reporter gene linked to MyoD-DNA binding sites. Physical inter action between Mos and MyoD, but not with E12, is demonstrated in vivo by using the two-hybrid approach with C3H10T1/2 cells and in vitro by using the glutathione S-transferase (GST) pull-down assays. Unphospho rylated MyoD from myogenic cell lysates and/or bacterially expressed M yoD physically interacts with Mos. This interaction occurs via the hel ix 2 region of MyoD and a highly conserved region in Mos proteins with 40% similarity to the helix 2 domain of the E-protein class of bHLH f actors. Phosphorylation of MyoD by activated GST-Mos protein inhibits the DNA-binding activity of MyoD homodimers and promotes MyoD-E12 hete rodimer formation. These data support a novel function for Mos as a me diator (coregulator) of muscle-specific gene(s) expression.