Neuregulin-1 induces acetylcholine receptor transcription in the absence of GABPalpha phosphorylation

Localization of acetylcholine receptors (AChRs) to the postsynaptic region of muscle is mediated in part by transcriptional mechanisms, because the genes encoding AChR subunits are transcribed selectively in synaptic myofiber nuclei. Neuregulin-1 (NRG-1) is a synaptic signal and induces transcription of AChRs in muscle cells. Signaling by NRG-1 is thought to involve the transcription factor GA-binding protein (GABP), a heterodimer of GABPalpha, which is a member of the Ets family, and GABPbeta. Phosphorylation of certain other Ets proteins outside the Ets DNA-binding domain serves to stimulate transcriptional activation in response to extracellular signals. According to previous studies, NRG-1 stimulates phosphorylation of GABPalpha at threonine 280 in the N-terminal region adjacent to the Ets domain, suggesting that GABPalpha phosphorylation might contribute to NRG-1 responsiveness. To determine the functional importance of the N-terminal region of GABPalpha and whether its function is regulated by phosphorylation, we generated muscle cell lines in which the endogenous GABPalpha gene was deleted and replaced by variants of GABPalpha mutated in the N-terminal region. We found that NRG-1 can induce transcription in cells with mutated T280 phosphorylation site, indicating that T280 phosphorylation does not contribute to NRG-1 responsiveness. We also found that NRG-1-induced transcription occurs in cells missing the entire N-terminal region of GABPalpha. Because NRG-1 signaling is not expected to alter the function of the C-terminal region, which remains in these cells, these results suggest that GABPbeta, or other interacting components, rather than GABPalpha directly, is targeted by NRG-1 signaling.