Key result
A 23-amino acid motif (residues 60-82) of zebrafish Myf5 is necessary and sufficient for nucleolus targeting, and Myf5 specifically binds to nucleolin/C23 unlike MyoD.
The distinct functions of Myf5 and MyoD may result from their differential binding affinity to nucleolin/C23, driven by a specific 23-amino acid motif in Myf5.
May underlie Myf5-MyoD divergence in myogenesis; leaves open mammalian validation and clinical relevance.
Myf5 is a nuclear protein and one of the basic helix-loop-helix (bHLH) myogenic factors that play an important role in muscle specification and differentiation. The motif responsible for the nuclear translocation of Myf5 was unknown. Using on-line monitoring of EGFP (enhanced green fluorescent protein)-tagged zebrafish Myf5 translocation, we demonstrated that Myf5-EGFP protein resided in the nucleoplasm and nucleolus of zebrafish fibroblast cell lines (ZEM2S and ZF4), mammalian nonmuscle cell line (COS1), and muscle cell lines (RD and C2C12). In contrast, zebrafish MyoD-EGFP was localized in the nucleus but did not condense in the nucleolus. Using indirect immunofluorescent staining, we determined that zebrafish Myf5 was colocalized with nucleophosmin/B23, a nucleolus protein. Deletion analysis revealed that amino acid residues 60 to 82 (60KRKASTVDRRRAATMRERRRLKK82) of Myf5 were sufficient and necessary for nucleolus targeting. A GST pulldown assay followed by Western analysis showed that nucleolin/C23 could be pulled down specifically by GST-Myf5, but not by GST-MyoD. Based on these findings, we propose that the distinct functions of Myf5 and MyoD may result from their differential binding affinity to nucleolin/C23.
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Wang et al. (2005) studied this question. Myf5 vs. MyoD was evaluated on Nucleolus targeting and protein interaction. A 23-amino acid motif (residues 60-82) of zebrafish Myf5 is necessary and sufficient for nucleolus targeting, and Myf5 specifically binds to nucleolin/C23 unlike MyoD.
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